Imagens das páginas

tances increase; and although the mass may be large, yet, from the effect of depth or distance, the indication may be too slight to be observed, unless by the most delicate instruments, skilfully used. By means of these, we may be guided to vast mineralogical treasures; for, however desirous we may be to discover gold and silver mines, iron is the more useful metal. We have discovered, in Iowa, one magnetical node, which may be produced by a “subterraneous iron mountain;" but of this, more hereafter. Independently, however, of any economical views, it will be a matter of gratification to the scientific world to receive a small contribution to their fund of magnetical knowledge; for an effort is now making to collect and imbody as many accurate magnetical observations as possible, in order the more fully to determine the changes, distributions, and general laws of this wonderful force, and to make it still more subservient to the purposes of general utility.

A very interesting report on the subject of magnetical observations has lately been made to the Royal Society, by Sir J. F. W. Herschell. Upon the approval of that report by the society, a deputation was requested to communicate certain resolutions to Lord Melbourne, and to urge on the Government the adoption of the measures therein proposed. “This,” says the editor of the Journal of the Franklin Institute, “has produced its de. sired effect upon her Majesty's ministers, who have appointed three officers of artillery, with adequate attendants, to go, respectively, to Montreal, to the Cape of Good Hope, and to St. Helena, to make experiments on the variations and dip of the needle, and the intensity of magnetism, frequently and simultaneously, each day, for three years,” &c.—(See table, p. 134.)

Instruments used in making the magnetical observations. The dip was observed by a dipping-compass made in 1837 by Robinson of London, on a plan similar to that upon which he has constructed them for Captain Ross, nephew of Sir James Ross, and for several other experienced observers; two needles were used at each station, and reversals made, both of the face of the instrument, the face of the needle, and of the polarity of the needle, by · retouching” upon the field with two magnetic bars. This mode of operating requires eight annotations of the dip with each needle—sixteen in all; and the results given in this report are, in almost every case, the mean of the sixteen. So well did the instrument perform, that the separate results of each needle differed not more than a minute from each other. The following is a copy from my field-book of the observations to determine the dip at Mineral Point, Wiskonsin, on November 5th, commencing at 9 A. M. NEEDLE No. 1-B North.-E. E. 74° 30.0'

W. W.72 10.5
W. E. 74 22.0

E. W. 72 22.5
A North.-E. B. 72 31.5

W. W.74 10.0
W. E. 72 12.5
E. W. 74 26.5

8)586 45.5
Mean for No. 1 = 73 20.6875

[ocr errors]

NEEDLE No. 2-A Norlh.-E. E. 73° 26.5

W. W. 73 9.0
W. E. 73 18.5

E. W. 73 12.5
B North.-E. E. 73 33.5

W. W. 73 15.0
W. E. 73 27.5
E. W. 73 21.5

8)586 44.0 Mean for No. 2 = 73 20.5

Here it will be observed, that, although needle No. 1 is unbalanced, and reads very wide of the mean on both sides, yet the errors are all merged by the reversals; and the mean by each needle does not differ from the mean of the whole the tenth part of the minute of a degree.

Mean by No. 1.-73° 20.6875
Mean by No. 2,473 20.5000

[blocks in formation]

The HORIZONTAL INTENSITY was determined by vibrating three needles* successively in a glass vessel exhausted, as near as possible, by a small air-pump, and noting the time required for five hundred vibrations by a chronometer, and the temperature by an included thermometer. The degree of exhaustion indicated by the mercurial ganige was from five-tenths to seven-tenths of an inch. The initial arc of vibration of each needle was about seven degrees on each side of the meridian, and the terminal arc a little over one degree. This apparatus for vibrating the needle, to deler. mine the horizontal intensity, was invented by A. D. Bache, president of Girard college. The final reduction for temperature, and the calculations for obtaining the numerical expressions for the horizontal and total intensities, were made according to the usual rules laid down for that purpose. (See President Bache's papers in the Transactions of the American Philosophical Society.) The numbers are, however, merely comparative, and have been referred to Louisville, Kentucky; the horizontal intensity of which, as taken at Corn island, has been assumed as unity, or rather as ten. It was intended to have compared the intensities with those taken at Cincinnati ; but it was found, from some cause or other, that the separate results obtained there by the three needles were discordant, while at every other station they agreed very nearly. I hope soon to be able to connect this group of observations with those made at Philadelphia, and thus make them comparable with the extended results oblained by the philosophers of that city and of foreign countries. It is true, I vibrated the identical needles

• Two of these were of the form and size recommended by Hansteen; and the third was a lat bar, with lozenge terminations. o

used on this survey at the Greenwich observatory in 1837, and obtained a comparison between the vibration there and at Cincinnati ; by which it appeared that the horizontal intensity at the former place was, to the horizontal intensity at the latter place, as 1000 is to 1163.6; but the needles were new, the observations with each necdle only once performed, and myself but little experienced in the manipulations.

The VARIATION was observed by me with an apparatus in no way superior to that used by our land surveyors. The few observations which I made correspond remarkably well with the returns of those surveyors.

The accompanying 6 variation chart” exhibits the variation at which the township lines have actually been run. It was compiled at the land office at Dubuque, from the field-notes, and kindly forwarded to me by my friend E. Dwelle, first clerk in that office.

By the chart exhibiting the dip and intensities, it appears that the dip does not increase regularly, or by an equal number of minutes, to an equal number of miles of north latitude. From St. Louis to Mineral Point the average increase is one minute of dip to 1.28 mile, or a little more than one mile and a fourth of a mile; but from Davenport towards Prairie de Chien, where the observations are sufficiently near to each other to exhibit local fluctuations, they are made very evident. From Davenport, northwardly, about thirty miles, it increases with great rapidity; aster which, it actually diminishes. It then increases rapidly to Dubrique. From Du. buqne to Prairie du Chien, the increase is very moderate; so much so that

the interval from 73° to 73° 20' is more than twice as great as the interval ' required for the previous increase of 20 minutes.

The LINES OF EQUAL DIP traced on the chart do not imply any great precision or exactitude. After the chart was filled up with the precise points of encampments, and the exact dip observed at those points, these lines were drawn with a free hand, so as to include, between any two contiguous ones, all those observations limited by 20 minutes. Proportional points were often taken in a line between two observations, and the lines of equal dip projected through them. These conditions of their consistency with the observations made it necessary that some of them should be curved much more abruptly than the projections of great circles. Towards the northeast part of the map, it will be seen that the lines of equal dip incline, in proceeding eastward, very much to the south; or, in other words, the dip increases in travelling eastward, as well as northward. This is remarkably evident in the journey from Prairie du Chien to Madison, nearly due east; and yet the dip increases four-fifths of a degree.

On reviewing the chart, it appears to me, from the few observations. I was enabled to make with reference to the localities of iron ores, that the rapid increase of the dip over certain regions, as between Davenport and Dubuque, and again between Prairie du Chien and Madison, corresponds to an increased quantity of iron in those regions. But this is a point which can much better be determined by yourself; as, by the numerous - specimens brought to you from every part of the surveyed region, you will be able precisely to point out the ferruginous tracts.

[ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors]

The most remarkable magnetical phenomenon developed by this sur. vey is a point of local attraction on the river Wapsipinecon, nearly

in the centre of the great bend of the Mississippi river, from Dubuque to Davenport. At this place all the elements of magnetism are suddenly-changed; the variation, the dip, the horizontal and total intensity, are all peculiar; the dip and total intensity are greatly increased, while the east variation and the horizontal intensity are diminished. On three different sides, the dip is less than at this point; thus it would seem there is a local point of dip, a magnetical island, not connecting itself by any line of equal dip with any other points; the proper line of 722 50 being about twenty-eight miles to the north of this place. At this point on the Wapsipinecon river was found abundance of iron ore, especially the hematite, which is a peroxide of iron, and does not in small masses affect the magnet; we found also detached masses of iron ore, not only magnetic, but actually magnetized, having permanent polarity and being proper loadstone. These were, however, too inconsiderable in mass, of themselves, to have produced the magnetical disturbance occurring here; there is probably, at an unknown depth, a mass of magnetic iron ore in this place—a subterranean “iron mountain” like those in Missouri: it may lie too deep for exploration. A more particular magnetical surrey might determine more precisely its axis, and point out the exact place for boring, or for sinking a shaft, with the hope of reaching the ore in place.

Dip and intensity in the lead-mines of Dubuque.

As the lead appears to be mostly associated with ferruginous clays, and not unfrequently to lie in contact with thin veins of hydrated peroxide of iron, I was desirous of making observations in the midst of a vein of lead ore, in order to see whether any effect would be produced by the contiguous minerals. After having observed the elements of the dip and intensity at the promontory of Dubuque's grave, I descended one hundred feet into the mine of Mr. Dougherty, (who kindly afforded erery facility,) and there repeated the observations; the results were identical with those of the former one, the metallic vein appearing to exert no peculiar magnetical influence.

It appears, by both the table and chart, that the total intensity increases in travelling towards the north, or, more properly, in the direction of north several degrees east-being least at St. Louis, and greatest at Madison, in Wiskonsin; the progression is not uniform from one of these points to the other, but undergoes several undulations. The intensity at St. Louis (29.366) is to that at Madison (30.433) as 1000 to 1036.6. The magnetic elements of direction and intensity are undergoing a slow and progressive change, and are also subject to annual and daily oscillations, taking place at particular seasons of the year, and at particular hours of the day. Besides these fluctuations, which have been ascertained and can be predicted, late experiments show that there are irregular fluctuations of small amount, which cannot as yet be foretold. In the foregoing observations and calculations, these varying quantities have not been taken into account.

In 1819, it was ascertained by Major Long's party that the dip at St. Louis was 70° 30'; it is now (1839) 69° 31'; showing a mean annual diminution of 3' 3".

It will be seen by the chart of variation, that the direction of the needle, as returned by the surveyors, undergoes numerous irregular changes, the effect of “local attractions.I had doubted the correctness of their observations, but I am fully convinced that these changes are at least as great as they represent them. Part of the region traversed had been surveyed by Mr. Burt, with his solar or equatorial compass—an ingenious contrivance, which measures constantly the sun's azimuth, and indicates the variation at any time of the day, during clear weather, by a solar spectrum; this presumes the latitude and sun's declination to be known, and the instrument to be adjusted accordingly.

I have already anticipated one of the uses to which a knowledge of the elements of terrestrial magnetism may be applied, viz: the discovery of some species of iron ores. It remains to make a few observations on a prop. osition which has been agitated for two years before the Congress of the United States, viz: to use the elements of magnetism as the means of determining the latitude and longitude for nautical and geographical purposes. The agitators of this proposition seem to have taken it for granted that the magnetical lines, meridians, equator, poles, &c., bear the same relation to the terrestrial lines and points of the same name, as the ecliptic and its secondaries bear to them; being that of two concentrical spheres, whose axes cross each other at an angle of 23° 28'. This should have been called a theory, not a “ discovery ;" for it contradicts, directly, well-observed facts, and disagrees with the observations recorded in this paper. Should Congress see fit to send the projector of this theory, or any qualified person, abroad to make magnetical observations, and make an appropriation for that purpose, it would meet the approbation of the learned throughout the world. But, that a splendid speculation should be agitated by persons who, however well qualified as men of literature and as politicians, are not adepts in the modern science of magnetism and electrology—that an amount of national treasure sufficient to have erected a magnetical observatory, should have been spent on a project which every magnetician in the country foresaw was absurd-are subjects much to be regretted, and are calculated rather to injure our national reputation for physical science. On this subject, I concur fully in the sentiments of Dr. Patterson and Mr. Walker, published in the Proceedings of the American Philosophical Society, for July and August, 1838.

I may be called upon to specify what I have charged in general terms; there is no end to words, and this is no place to go into a long argument.

It is asserted in thal theory, that "the line of no variation is a great circle of the earth.” Those who have actually surveyed that line, report it as being very irregular, and not a great circle. The lines of equal variation, as observed and reported in the returns of the surveyors of western parts of the United States, proceed southwardly, and curve rapidly to the west. If, as observations seem to indicate, the elements of magnetism are subject to all of the changes previously named, being affected by progressive changes, by summer and winter, by day and night; and, also, like the barometer, by irregular, unforeseen causes; they will, for the present, scarcely answer as a substitute for astronomical observations in determining geographical position.

« AnteriorContinuar »