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report on the history and progress of the american coast survey up to the year 1858


REPORT ON THE COAST SURVEY

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TESTIMONIALS FROM THE MERCANTILE COMMUNITY TO THE VALUE OF THE COAST SURVEY.– Should we aim at conceiving a definite estimate of the value of the contributions afforded by the Coast Survey to the development of the commerce, and thereby to the general prosperity of our country, we shall find the importance of these laboriously-gathered results so to grow upon our minds, as we consider them, as to baffle all attempts at computation.

The amount of wealth (and not less of human life) which they have saved from being ingulphed in the waves can never be known. The degree to which marine commerce, relieved by them of many of its perils, has been stimulated into higher activity through their influence, must be equally matter of conjecture: so that, in fact, the difficulty of forming any clear or adequate idea of these great benefits, grows mainly, at last, out of their very magnitude.

When men, like ourselves, whose life is spent upon the land, and whose occupations little familiarize them with the very-day operations of commerce, have their attention more particularly drawn to this subject, they may begin to feel by degrees its practical importance, and in some measure to realize the far-reaching consequences of the labors we are reviewing. But it is far otherwise with the merchants and the sea-faring men of the country. Truths which we recognize but occasionally, and by a special effort of the attention, are their daily and familiar thoughts; and thus, whenever an uttered opinion in regard to the operations of the Coast Survey has been heard from such a source, it has invariably been an emphatic testimony to their value.

Thus, the Insurance Companies of Boston aver, that “being constantly concerned in the security of the navigation of our own shores, and having watched with interest the condition and progress of the Coast Survey,” they “are satisfied that every year it is rendering very valuable service to the commerce of the country by its accurate charts and by its important discoveries;” and they express their conviction that “the present organization of the Survey is better suited than any other to insure the prosperity of this great national work, and to provide the best judgment in directing its plans of operation, and the greatest skill and fidelity in their execution.”

The Chamber of Commerce of Charleston “considers this work as if more importance to Charleston than any work that the General Government has ever undertaken for it.”

The Savannah Chamber of Commerce, considering it “matter of great interest to the people of Georgia that the work which has been for some time past going on in the survey of our coast should be prosecuted with vigor,” resolved that their senators and representatives be “respectfully requested to aid the grant of the necessary appropriation.”

A meeting of citizens of Mobile “cordially recommend to the favorable notice and action of Congress the continuation of an energetic and active prosecution of the Coast Survey, as a work of the greatest utility to the various interests of the Union, and calculated to reflect honor upon the intelligence and patriotism of this country,” and that their thanks be “tendered to Professor Bache and the officers under his direction, for the zeal and success with which their duties have been performed in the bay and harbor of Mobile.”

Still more recently than the before quoted testimonial from New York, appears a memorial signed by the most influential merchants, ship-owners and underwriters of that port, who, “Believing that the means of safety are increased by the vigorous prosecution of the United States Coast Survey, from its continually developing new dangers, and locating them so that they may be avoided, do respectfully ask that the Secretary of the Treasury may urge upon Congress the necessity of increased appropriations for the purpose of bringing the work to a more speedy completion.” Also appears a memorial addressed to the present Honorable Secretary of the Treasury, and signed by underwriters, merchants, traders, manufacturers, mechanics and business men of Philadelphia, who state that they “have for many years witnessed the satisfactory progress of the survey of the coast of the United States,” and that “this great work has already proved of eminent advantage to the commerce and navigation of the whole country, by adding to the safety of life and property” and that, “believing that the benefits to be derived from the active and extended prosecution of the Coast Survey are of a character to reflect honor upon the administration which cherishes and supports it, and that we should fail as a people in our duty to the world by neglecting to acquire the most accurate knowledge of the dangers that lie upon our extended line of sea-coast and harbors,” they “most respectfully ask the support of the Coast Survey in” the Secretary’s “annual report, and the recommendation of ample appropriation for its use.”

SEA-COAST CHANGES.— A careful examination of the list of discoveries above given will bring to view a fact which, to a person unfamiliar with this subject, may occasion considerable surprise. More than one in ten of the discoveries of the Coast Survey are discoveries of changes in the ocean’s bed; not only among shoals at sea, but also within some of our most frequented harbors, and in the channels by which they are approached. Similar changes occasionally occur, also, even in the configuration of the visible coast line itself. This consideration is sufficient to show that the hydrographic part of this great work cannot, like the geodesic, be, once for all, completed; but that, in order to maintain permanently in the charts the same degree of accuracy which is originally given to them, the soundings must, from time to time, be revised, and all the changes of depth discovered, carefully noted.

TIDES AND CURRENTS.– Another fact, which the attentive study of the same catalogue discloses, is, that an acquaintance with the ocean currents, and with the irregularities of the tides, is of almost as much importance to the mariner as a knowledge of the depth of water, or of the form and character of the bottom. These things, and, in addition to them, the magnetic elements, are, in some cases, for the sake of greater clearness, given in separate charts. The present Superintendent of the Coast Survey has devoted earnest attention to all the subjects of this class; and he is especially developing the complicated laws which govern the tides on our coast, in a comprehensive and masterly manner. The preliminary chart of cotidal lines on the Atlantic coast, and the tide-tables constructed from the observations of the survey, have been already incorporated into Blunt’s “Coast Pilot;” for, although the whole investigation is still in progress, and is necessarily, by its nature, far from its completion, important practical results have been developed which have been immediately put within the reach of the navigator. Preliminary charts of tidal currents have also been prepared for various localities at the office of the survey.

Practically valuable, however, as is the investigation of this difficult subject, which, in the present able hands, is proceeding so successfully, it is of the highest interest also in a purely scientific point of view. The problem is one of the few that belong to the globe as a whole; and to us, as possessing a territory which borders on the two great oceans, the solution of the problem appears to be more especially committed than to any other civilized nation.

The subject here touched upon, sea-coast changes, tides and currents, while, in their practical bearings, as they affect the interests of commerce, they present no aspect which cannot be equally comprehended by every mind, have yet a higher significancy and stand in a broader relation to the physical history of the globe than is likely to occur to any but those who have made the operations of nature, as they are constantly proceeding on the largest scale over the whole surface of our planet, the subject of long-continued and careful study. The opinions of such men, in regard to the value of observations like those which have just been occupying us, are of much more than ordinary interest; and, on this account, the following, from a member of this Committee, whose life has been devoted to labors which make him a high authority upon all maters of this description, is given in his own words:

“The observations on the action of tidal and river agencies, in changing the form, depths, and entrances of harbors, are of well-known importance. These changes are so perpetually in progress, and in time become so disastrous to navigation, that a corps charged with the duty of surveying the coast, and mapping out the annual variations of sand-banks, headlands, and channels, is part of the necessary organization of a well-ordered government. But beyond this, the Coast Survey, as at present conducted by Prof. A.D. Bache, is furnishing– in addition to the most exact determination of all the features of harbors, and the special causes of change operating in each, with the progressive variations in the action of these causes themselves– that comprehensive survey of the whole continental coast, and the movements of the bordering ocean, which is essential, in order to arrive at the general principles governing tidal operations and coast changes. It is mapping out the whole grand subject of the Atlantic tides so completely, that the special details of each bay and inlet are taking their true places in the system; and subordinate to these, also, the movements and counter-movements in harbors, which occasion their changes in depth and outline. The questions that must often come up respecting the improvements of harbors, or their protection from encroaching silt or sand, are thus receiving thorough solution by reference to the widest and most fundamental principles.”

The Committee here perceive that they are approaching the limit at which the practical becomes blended with the scientific; and while those results of the survey which remain to be examined are by no means deficient in value, nor often even in a value eminently practical, yet, since they cannot properly be classed among the explicit objects in view of which the work was originally instituted, their consideration appears most fittingly to fall under the head of Collateral Results.

COLLATERAL RESULTS:— ADDITIONS TO THE AMOUNT OF SCIENTIFIC OR USEFUL KNOWLEDGE

CONTRIBUTIONS TO GEOLOGICAL KNOWLEDGE.— All observation conspired to instruct us that the surface of our earth, under the action of the elements, is undergoing continual changes; and that the relation existing between land and sea is in a state of incessant oscillation. What the Coast Survey has disclosed to us, by its severely exact methods of scrutiny, of the mutual and steadily progressive encroachments of the water and the land, along the entire extent of our own ocean border; that, also, successive generations of men, observing without method, have, in the lapse of centuries, been compelled to recognize, as a law of universal prevalence, wherever, throughout the glove, these contending elements come into contact; and that, too, many facts of recorded history attest, in instances which the tooth of time has so completely obliterated all natural traces of the former state of things, that, but for the existence of the record, not a suspicion of change could ever have been entertained. That, also, many imposing monuments of ancient art, colossal mementoes of forgotten centuries, once towering sublimely above the pigmy dwellings of their builders, but now half-buried in encroaching sand, avouch in solemn silence to the curious wanderer of the desert: while other ruins, no less interesting, tottering on the margin of the sea, reveal the same truth in the deep-worn evidences, which they bear far up among their crumbling sculptures, of former watery violence; and in the still more wasting ravages of the lithodomous molluses which burrowed in their substance when the waters, which vainly lash their foundations to-day, enveloped them, in some unknown period of the past, half-way up to their elaborately wrought capitals. That, also, islands upheaved above the ocean’s surface within the period of authentic history, or cities, within the same period, submerged beneath its waves– extended lines of coast which the paroxysmal throes of volcanic energy have suddenly elevated many feet above their former level, or as suddenly depressed below,– or which the steady and long continued progress of secular change has lifted or submerged more slowly; that, also, the growth or wasting of the shores under our own eyes, in numberless localities where no change of the general level is perceptible; that, also, the rounded and water-worn forms of the pebbles which lie imbedded, in countless multitude, in the superficial strata of every continent,— and the vast deposits of marine organisms which crown the very mountain-tops with the evidences of once swarming primeval life; that, in short, an almost endless series of unequivocal and most convincing indications proclaim, in modes as various as the mute witnesses themselves, but with a harmony of testimony as irresistible as it is impressive.

Considerations like these invest the researches of the Coast Survey with that species of fascinating interest which attaches to mystery, and impart to them a portion of the dignity which belongs to those grand operations of nature, whose laws they assist us to unveil. This noble work is gathering for us the statistics of secular progress; and it is no idle imagining to suppose that, out of its systematized results, and those of other equally searching investigations already in progress, or hereafter to be instituted in other lands, science may yet be able to unfold to us all the wonderful history of those stupendous revolutions which have marked the past, and all the inevitable transformations which still impend over the unknown future of our globe. Upon the point here presented, the Committee take the liberty to quote once more the language of the eminent authority already introduced.

“Geology is gathering much from the results of the Coast Survey. The rocks of the globe have been formed, to a great extent, through tidal and current action, many of them having been accumulated, just as sand and mud deposits are now accumulating along and off the shore from New Jersey south, or as deltas are forming about the mouths of rivers. The operations of the present day thus explain those of the earth’s past history. Science is hence deriving from the researches of the Coast Survey, facts elucidating the whole series of geological formations. The surveys of the delta and mouths of the Mississippi, of the harbors of Mobile and Charleston, of Delaware river and bay, of New York and other bays and harbors, are all subjects for profound study with those who would understand this great branch of geological dynamics; and they look for other important data from the future surveys, that shall exhibit the changes that are now in progress, and the rate at which they are going forward.

“Besides investigating the origin of sea-shore deposits, and the causes of their varying features, soundings off the coast and in the deeper waters are making the sea-bottom a part of terra cognita, disclosing the positions of its valleys and plains and the character of its slopes, and marking out the true limit between the continental areas and the great oceanic depressions. The coast Survey is thus enabling the geologist to extend the area of research, compare the causes and characters of submarine ridges with those of the sub-aerial, and trace the directions of river channels over the sea-bottom off their mouths.

“By its registers of depth, it is also preparing to detect evidences of variation of level along the border of the continent; or evidences of stability, as the case may be: and this is becoming a subject of practical importance. Suspicions have been thrown out respecting a slowly progressive subsidence along the coast of New Jersey, which, if sustained by a thorough examination of the facts, suggest a new topic for serious consideration to all interested in New York commerce. Whether true or not, the point requires examination for every part of the coast of the United States; and this examination it has begun to have, in the accurate system of soundings and measurements of the Coast Survey.

“Along the coast of southern Florida, the subject of coral reefs comes under the attention of the Survey; and in investigating the forms and formation of coral-reed harbors, like that of Key West, their rate of filling up, or the influence of currents in keeping them open, geology is receiving other important contributions from the facts which its operations are disclosing.

“It may finally be observed, that the study of the material brought up by the lead, in the prosecution of the deep-sea soundings, is making some remarkable revelations with regard to the distribution of animal life; and that thus zoology and the geography of life are deriving much from the investigations in progress.

“All these scientific contributions are collateral results, incidental to the great purpose of the survey– results attained, not through independent researches needlessly introduced, but because they are of necessity involved in the prosecution of the great purpose itself, and thus constitute a part of the evidence demonstrating the strict fidelity and profound ability with which it is carried out. They flow from the work as its entirely legitimate fruits, and take their place, of unquestionable right, alongside of those other fruits whose legitimacy is universally recognized, and in which humanity, as well as commerce, is so deeply concerned.”

As somewhat more specifically illustrative of the nature of the interesting inquires connected with this subject, which the Coast Survey is solving, or will yet be able to solve, the committee venture to quote, in addition to the foregoing, the subjoined passage from a communication obligingly furnished by another geologist, standing deservedly high among the scientific men of the country:

“The Mississippi river,” observes this gentleman, “presents, perhaps, the most magnificent example in the world of a large river discharging abundant sediment into an almost tideless sea. In the Gulf of Mexico, therefore, we have the best opportunities of studying the phenomena of river deposit. The distribution of the various kinds of sediment brought down by the great river, the slope of the submarine bank, destined to be gradually transformed into delta and added to our territory– these would be problems worthy, in themselves alone, of the attention of an enlightened government. And these important problems have not escaped the attention of the accomplished superintendent; as may be seen in the report for 1856, map No. 40, where the distribution of sand, mud and coral-mud is carefully laid down. But there are several points of interest which deserve further attention: It will be seen, by reference to the map just mentioned, that the lines of equal depth approach each other near the mouth of the Mississippi, showing that the submarine bank formed by the deposit of this river has a very rapid slope toward the south. Near the mouth of the river runs the line of six fathoms; about ten miles south, the line of fifty fathoms; and from this point the bottom slopes so rapidly that, at the distance of three or four miles more, a line of two hundred and thirty-six fathoms found no bottom, making a difference of nearly eleven hundred feet in this short distance. Thus we have a considerable inclination of strata over large areas, produced by the natural agency of water alone. It is possible, however, that the steepness of the slop may be, in part, due to the existence of currents (e.g. Gulf-stream,) sweeping across the mouth of this river at some distance, and carrying away sediment which would otherwise be deposited further out. It is important to ascertain how far this southward slope is modified by the existence of any such current.

“The same map shows that, on the Gulf side of the peninsula of Florida, the coral bottom (probably coral mud) extends as far north as Tampa bay. It would be extremely interesting to determine whether there is any return current carrying coral mud from the southern point and keys of Florida, northward, or whether, as is much more probable, we have here additional evidence of the fact that the coral formation of the peninsula extends northward as far as Tampa. It will be recollected that, according to Tuomey, the eocene may be traced as far south as this point. It would be important to trace, in a similar manner, the coral deposit on the east side of the peninsula as far as possible.”

Were it thought indispensable in this report to pursue a method severely logical, it would be proper here to complete whatever remains to be said in regard to the aid which the survey of the coast is rendering to geological science. Inasmuch, however, as the formation of submarine strata depends, as we have seen, in great measure upon the distribution of sedimentary matter through the agency of currents, and inasmuch as the greatest of all the currents in our waters, or indeed the waters of any known sea, has been a subject of special study and elaborate exploration, under the direction of the superintendent of the Coast Survey, it has seemed convenient to defer the consideration of the relations of that great stream to geology until some account shall have been given of the character of the observations made upon the stream itself, and of the results which those observations have gathered.

EXPLORATION OF THE GULF-STREAM.— The observations made in the progress of the survey of the coast, with regard to the currents of the ocean, and particularly with regard to the Gulf-stream and Labrador current, are in point of importance not inferior to those made on the tides. The oceanic currents embrace completely the whole globe, and follow as regularly an established system in their movements as the blood circulating in a living being. This, therefore, in its most general aspects, is one of those world-wide subjects which nations should undertake to investigate; but, in the actual discharge of the duty thus resting on them, it is fitting that each nation should explore, with special attention, the seas that wash its own shores. To ourselves, whatever relates to the Gulf-stream possesses a paramount interest, since every vessel which approaches our Atlantic coast from abroad is compelled to cross this great river in the sea, in the exploration of the stream which the Coast Survey has undertaken, the velocity and direction of the current at all points are, of course, primarily noted; but especially is the temperature of the water made a subject of very careful study in all parts of the stream. The distribution of temperature in the axis of the stream, at different depths, and in sections at right angles to the axis, has been diligently investigated, and is exhibited, so far as it has been hitherto ascertained, in preliminary charts. The investigation embraces, also, the position of the axis of the stream at different seasons of the year, with the variations in its breadth, the changes in the distribution of temperatures at different seasons, and the connection of the figure of the bottom with this distribution; also, the characteristics of the inner cold current making in a direction opposite to that of the Gulf-stream; all of which points are materially interesting, not only to general science, but to practical navigation. With the aid of the perfected map of this stream, and of its attendant counter-current, such as the survey will eventually bring forth from its accumulating materials, the navigator approaching our coast will obtain most important indications of his position. The color of the water itself will be one of these; but a more interesting one will be found by merely consulting the temperature of the sea, extending the observations to different depths below the surface, as well noting the superficial temperature. Thus the remarkable and sudden change of temperature which occurs along the inner edge of the current, the warm water of the Gulf-stream being confined as it were by a “cold wall” of water (a term happily applied to it by Lieut. G.M. Bache) is a valuable practical guide; yet, in order that it may be a safe one, the laws which govern the position of the “cold wall” at different seasons, must yet be carefully determined. This, and various matters of interest still unsettled or obscure, can be properly developed only by a series of observations, extending over a number of years. The Coast Survey is, in fact, now supplying what Humboldt, so long ago as the year 1804, clearly set forth as important desiderata in relation to the Gulf-stream– a subject to which he had devoted much careful attention. [ Personal Narrative, &c., chap. I.]

We may, at this point, with propriety resume the subject, deferred in the discussion fo the topic last considered, of the relations of the Gulf-stream to the submarine deposits heretofore formed, or still in process of formation, upon our shores and beneath the waters of the neighboring seas. The formation of shore deposits, whether in tideless or in tidal seas, has been investigated to some extent elsewhere, particularly under the direction of the English government, in the British seas and the German ocean; but of the deposits of Oceanic currents in deep seas, we know nothing but what has been revealed by the Coast Survey.

One of the most important contributions by which American geology has ever been enriched, is the discovery made by Agassiz, under the direction of the Survey, that the peninsula of Florida is of recent origin; that it has been formed, in fact, by the growth of successive coral reefs, one outside of the other, from north to south; and that this progressive growth is still slowly proceeding. It is, however, the plausible suggestion of Prof. Joseph Le Conte, that, in order to furnish a foundation on which the corals might build, it is necessary that some previously acting cause should be taken into the account; inasmuch as probabilities favor the supposition that the water in which the present reefs are found has been, at a former period, too deep for the operations of these polyps. Such a cause he finds in the great stream we are considering. The waters of this stream, emerging from the Gulf of Mexico highly charged with sediment, and sweeping in a curve around the point of Florida, must, in his view, according to the laws of current action, have deposited a portion of their suspended matter within the curve. And thus, as he supposes, a submarine bank has been progressively formed, extending slowly southward, upon which the distinct reefs pointed out by Agassiz have been successively built. Whether or not this theoretic view is correct, is a point in regard to which geologists will look with interest to the investigations of the Coast Survey to determine.

Another interesting class of facts, associating itself naturally with the explorations of the Survey, is that which relates to the agency of the Gulf-stream in determining deposits in the open ocean. The velocity of this powerful current, as it rushes through the straits of Florida, is not less than five miles an hour; and with this great velocity it emerges to the northward into a wider sea. It is a property of currents, in such cases, to form an eddy on either side: and if the stream bear sediment, a bank is the necessary consequence. A notable instance of this is seen in the tidal current which rushes through the British channel into the German ocean, forming eddies which have resulted in the production of submarine banks. The Bahama banks lie east of Florida, separated from that peninsula by the Gulf-stream, and just in the position in which the existence of the stream would naturally lead us to look for such deposits. Upon these banks, also, coral reefs and keys have been formed. It is highly desirable that they should be further examined with reference to this point, and it is to be hoped that the operations of the survey may be so extended as to cover them.

The very remarkable configuration of the sea-bottom in and about the Gulf-stream, which has been so beautifully brought out by means of sections in the Coast Survey report of 1853 (maps No. 15 and 16) is a fact of great importance in a geological point of view. Commencing from the coast, the sea-bottom deepens very gradually, so that, at the distance of fifty miles, only the depth of twenty fathoms is attained. From this point, the bottom slopes rapidly down to very great depths. Thus the Gulf-stream runs like a river which has overflowed its banks. Now this submarine river-bank exists all along the coast side of the Gulf-stream. It is upon the margin of this bank that the coral reefs of Florida have probably grown. The bank may have been produced everywhere, by deposit from the Gulfstream; or it may be true that its formation has been the result of sediment from all the Atlantic rivers, deposited for a certain distance off shore, and then, as it were, cut off and carried away by the current of the stream. Both causes have, not improbably, been concerned in producing the observed effect; but the question is one which cannot be regarded as settled, and one on which the Coast Survey will yet probably shed much additional light.

The sections already spoken of, in addition to the lateral bank just mentioned, reveal the existence of enormous ridges and correspondingly profound hollows, running in the direction of the stream, and therefore nearly parallel to the coast. The stream is, moreover, divided longitudinally into several alternating bands of warm and cold water; the bands of warm water corresponding to the hollows, and those of cold water to the ridges. These bands and corresponding hollows and ridges commence at Cape Canaveral, and extend to a great distance; becoming, however, more indistinct as we go further northward. One of the cold bands runs along the lateral bank, and is that which has been already mentioned under the name of the “cold wall” of the Gulf-stream. In this, a slight return current has been detected; but whether or not the current is permanent, is a question yet to be definitely settled.

Now the origin or occasion of these cold bands is a question of the highest interest. It may be inquired, are they bands of Atlantic water, alternating with the water of the Gulf, or are they the cold water of a return polar current, deep-seated generally, but rising somewhat nearer the surface along these lines? It is very important, in view of these questions, that the direction of the current of the cold bands should be determined, and, if the warm and cold waters tend in the same direction, that their relative velocity should be ascertained.

It is, again, a question of no less interest than the foregoing, to what cause we are to suppose the alternating ridges and hollows to be due. Possibly they are mountain ranges, with their corresponding longitudinal valleys; constituting, thus, a submarine system parallel to the Appalachian chain. Possibly they are only lines of sedimentary deposit, determined by the existence of the cold bands of comparatively still water, or perhaps even of return currents. If the latter supposition be correct, then the ridges should decrease in height as we go north; for the sedimentary matter must sink, as it is borne further, to a lower and lower level. And even if the former be the true supposition– that is to say, if these elevated ridges are mountain chains of igneous origin– it is still possible that they may have undergone great modifications of form, from the subsidence upon them of the matters held in suspension by the waters of the Gulf stream.

The currents of the ocean suggest, by natural association, the movements of the more subtle medium above it– movements no less important to the navigator than those of the ocean itself, since the question of his safety is often dependent upon the degree to which they may be favorable or adverse. On this subject the Coast Survey has already accumulated much valuable information; and in its further progress it will unquestionably gather much more. It is, therefore, one which may appropriately claim the next place in this review.

CONTRIBUTIONS TO METEOROLOGICAL KNOWLEDGE.– It is obvious that a correct knowledge of the force and prevailing direction of the winds in different seasons of the year, especially in the vicinity of harbors, must be of the highest importance, not only of the safety of vessels and their cargoes, but to the security of human life. The physical causes which give rise to the great general atmospheric currents may be considered as satisfactorily established; but the influence of local causes in modifying these currents is yet very imperfectly understood. It is well known that the direction of mountain chains and river valleys frequently determines the course of the prevailing winds. This is strikingly exhibited in various places on this continent, and especially in the valley of the Hudson and Mohawk. And the analogous conditions which every line of coast presents, lead to the necessary conclusion that such modifications of atmospheric currents must be produced by them near the land, which must either be very serviceable or very dangerous to the navigator, according to the state of his information.

Observations bearing on this question have been incidentally made, in connection with the tidal observations of the Coast Survey, under the judicious guidance of the superintendent; and these are sufficient to throw some light on the laws which regulate the winds at certain points along our coast. At Fort Morgan, at Mobile point, and at Cat island light-house, near the entrance to Lake Borgne, the observations have given evidence of local peculiarities, at once interesting to science and important to navigation, distinguishing the winds which prevail in this part of the Gulf of Mexico. The remarkable frequency of north winds at this point, and the absence of west winds, so strikingly brought out in the graphic delineations of the superintendent, are probably in a measure due to the influence of land.

The effect of the winds, according to their varying force and direction, upon the level of the water in harbors and inlets, is another interesting result incidentally developed by the Survey, in connection with the investigation of the tides. The discussion of the observations made during the hydrographical survey of Albemarle sound, clearly indicates the influence of the wind on the height of the water, and has furnished the means of tracing several anomalous effects up to their true causes. A similar discussion of the influence of the wind on the tides in Cat island harbor on the Gulf coasts, leads to the same general conclusions, with differences in the numerical details, which are of high scientific interest. The observations conducted for some years past at Key West, Fort Morgan, and Galveston, and a careful investigation of the winds of the Gulf coast, made for purposes of navigation, and as determining the working season of the Coast Survey, have already led the superintendent to many valuable deductions; and it is highly desirable that similar observations should be made at every point commercially prominent along our ocean border. Such researches have an obvious bearing on the safety of navigations; and those which relate to the effects of winds on the waters of harbors and inlets are absolutely indispensable, in order to secure the necessary accuracy in the hydrography of the coast.

Other questions connected with atmospheric influences which the Survey may yet clear up, can be but hinted at. Such are, for instance, the effects of rain or drought upon the height of the water in these harbors, inlets and bays which receive the waters of extensive hydrographic basins, and the inquiry, how far the mean level of the ocean is affected at any given place by the pressure of the atmosphere upon its surface.

CONTRIBUTIONS TO OUR KNOWLEDGE OF TERRESTRIAL MAGNETISM.– Ever since the directive property of the magnetic needled became known, that little instrument has been regarded as an indispensable part of the outfit of every vessel navigating the seas. Its utility to the voyager is of so universal notoriety, that no argument is necessary to prove the value of any species of research whose result is either to add to the sensitiveness of the instrument itself, or to perfect that knowledge of the earths’s magnetic influences which is essential in order that its indications may be practically truthful or trustworthy. The use of the compass is to denote to the mariner the position of his meridian; but the needle itself very rarely coincides in direction with the meridian. What, therefore, the mariner must know, if he would not be misled by the very guide in whose infallibility he often for days together implicitly trusts, is the exact amount, in angular measurement, by which the visible indication of his instrument is fallacious.

Now, the magnetic meridian is, of course, laid down upon the charts. But this meridian is not fixed. Its position on the chart is true only for the time when the chart is constructed; or rather, for the time when the observations upon which it rests are made. It is therefore an important problem to determine, if possible, by what law of change the position of this meridian is affected, at every point of the coast. But it is entirely evident that this can be done only by comparing observations regularly made during a long period. It can be done, in short, in no other way but by that species of patient, and laborious, and protracted observation, carried on simultaneously at a large number of scattered stations, which, in the earlier part of this report, has been described as the method which the superintendent of the Coast Survey, with all that ability and knowledge of terrestrial magnetism for which he is distinguished, is actually pursuing. These observations are desirable, and even requisite, that they should be extended into the interior of the country, in order to develop the curves of equal declination (isogonic lines), or those, in other words, upon which the variation of the compass is the same. The determination of these curves by observation at sea, with the precision requisite for the development of the laws of change, is almost if not quite impossible; but from the curves on land, extended beyond the coast, all that can be desired will be finally attained. What has been thus far done is exhibited in valuable preliminary charts. It is hardly necessary to observe that the collateral investigation of the magnetic inclination, or dip, and of the intensity of the magnetic force, constituting, indeed, an essential rather than collateral part of any investigation relating to terrestrial magnetism, have been prosecuted pari passu with that of the declination.

The results thus gathered possess a double value; for while, to the mariner, they associate themselves with questions which involve his personal safety every day and every hour, to the philosopher who devotes himself to the study of nature in her mysteriously varied manifestations of force, they possess a curious interest which grows with the aliment it gathers, and sustains his zeal in quest of clearer light, under all the oppressive labor of observations multiplied without limit, and of reductions indescribably tedious.

But there is another species of importance attaching to the results elicited by the investigations we are considering, which recommends them to the attention and favor of citizens of every class. It has already been intimated that the direction of the magnetic needle is not only in general not due north, but that it is widely diverse in different localities, and never constant in the same locality. Near our extreme north-eastern boundary, for example, the deviation is at this time fifteen degrees to the west of north, while on the borders of the Pacific ocean it is no less than twenty-one degrees to the east of north. Between these limits it has every intermediate value. Now, were this state of things permanent, we might accommodate ourselves to the diversity, wide as it is; and when once the actual variation had been ascertained for every locality, no further inconvenience need be experienced. In the early history of this science, and before the fact had been detected to what an extent the declination of the needle is subject to change, it was regarded by the British Government as a matter of so high importance to ascertain the true amount of the declination in every part of the world, that the celebrated Halley was sent out in command of a government vessel, to circumnavigate the glove at the public expense, in order that he might gather materials for the construction of a magnetic map of the world. His mission was faithfully accomplished, and the required work was executed in a manner worthy of all praise; but hardly had his map been given to the world before the steadily progressive natural changes of the earth’s magnetic forces had vitiated all its indications, and rendered it, for the purposes intended, practically useless.
Now the manner in which these changes affected landsmen will require no explanation, when it is recalled to memory that all the boundaries of landed property are run out by the help of the magnetic needle. But the line which bore due north, by compass, or bore in any other specific direction, twenty years ago, or ten years, or even one, varies from that direction at present very considerably. When, therefore, a not very well informed surveyor (and there are a good many such in the country,) attempts to use these former bearings to fix the line-fences of a farm, he runs out a new piece of land, of precisely the same size and shape as the original tract, but with no single one of its boundaries coinciding with the original lines; so that the new piece of ground thus laid out overlaps the neighboring farm on the one side, and leaves a gore between it and the neighbor on the other side. Then comes litigation in its most perplexing form. But these embarrassments will be removed, for all future time, from all places included within the range to which the magnetic observations of the Coast Survey may extend. The Survey not only gives the actual variation at definite dates, but, by a beautiful application of mathematical principles, involving most abstruse considerations, it has deduced the law which governs the changes, and made it possible accurately to predict their future, and to reveal their past.

The Coast Survey reports for 1855 and 1856 contain tables of magnetic observations and charts of the magnetic declination, dip and intensity, in which the information thus far collected is presented in very perspicuous form. Discussions of the law of secular variation, with diagrams illustrative of the same, are also given there.

The true bearings between the Coast Survey stations being known, and published in the lists of Geographical positions (C.S. reports for 1851–53–55–57 ), it is a very simple matter for a surveyor, living in the vicinity of such stations, to ascertain the variation of his compass at any time, by observing the compass bearing of a line between such stations. In many instances the assistants in the Coast Survey have also planted meridian marks, at the request of local authorities, to facilitate the ascertainment of the variation of the needle.

BEARING ON GEODETIC SURVEYS OF THE STATES.– It has been seen that the ordinary methods of surveying are unsuited to the work of delineating the boundaries and territories of a state. In fact, that which is called a state map in our country is rarely more than a forced combination of county maps, these made up in turn from the surveys of townships, or else determined by arbitrary guess-work. It is obvious that the townships, however accurately surveyed by compass and chain, can never be represented truly on the composite map of a county; and that the maps of counties so made up cannot be brought together as to exhibit a correct view of the state. If the surveyor has assumed a southern angle as his point of departure, each northern line will be too long; and if his survey has begun at the northern extremity, all his southern lines will be too short. Errors similar in their result will attend the running of eastern and western boundaries; and at last, with all these errors, both of latitude and longitude, the lines of the survey must be cut down or extended, to bring them into some semblance of a relation with the equally erroneous lines of the divisions which are to surround it on the map.

It is within the knowledge of almost every land-surveyor in the older states, that the surveys under which titles were made from the first proprietors almost invariably overlap each other, or leave vacant gores and quadrangles between them. Nor has this wholly resulted from the changing variation of the compass, though we have seen that that cause may easily produce the effect; but from fundamental error in the principle on which common field surveying rests– error which rapidly opens out, as the area surveyed is enlarged. So notorious and general are, or have been, these conflicts of title, that courts have long since held that wherever natural, or even artificial landmarks can be found, the angles and lines of the record are to give place to them. Ley any one compare the modern trace of a railroad with a map of the state which it traverses, and he will find that neither the bearings nor the distances agree. Let him undertake to measure the farms which are contained within any one of the measured triangles of the Coast Survey, or, if he prefer it, within the local map of his own township, and he must not be surprised to find– for he assuredly will– that the whole is either greater or less than the sum of the parts.

In older countries more than in our own, yet quite enough even among us, this has been recognized as a serious evil, embarrassing titles, inviting litigation, and, in so far as such causes are operative, discouraging and impeding agricultural progress. The result abroad has been, that the government has assumed the burthen of ascertaining for every man the true boundaries of his landed possessions. The geodetic map of France marks down the lines of every farm as definitely as it does the site of a light-house, or the course of a stream. The ordnance maps of England are some of them on a minor scale; but their tracings are almost equally minute: while the more recent surveys of Scotland and Ireland are expanded to the proportion of an inch to the acre, and note even the minor subdivisions of fields and messuages– a scale not too large, where the revenues of government are derived in part from a tax on lands.


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