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DIURNAL VARIATION.

The following table of diurnal variation was sent to me by honorable William A. Burt, who is in the constant practice of keeping a meteorological table in connection with his observations on the magnetic variation; the results are the same as noticed in my former report, though not then in detail. These observations were made in lat. 42° 43′, N., long. 5° 24′ 30′′ west, from Washington.

Table of Diurnal Variation, taken in Macomb County, Michigan.

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Note.-July 19, at 12 h. 30 min. P. M., variation 1° 10'; at 12 h. 45 min., var. 1° 15'; at 1 h., var. 1° 28'. July 24, at 6 h. 10 min. P. M., shower commences, var 1° 35'; at 6 h. 40 min, shower past, var. 1° 25'. July 27, at 5 h. 45 min. P. M. shower rising, var. 1° 47′; shower past, var. 1° 37'. COUNTY SURVEYORS-law respecting them-Judge Burt's compass-magnetic meridians-disappearance of original lines. From the statute* regulating the duties of county surveyors, it becomes indispensable to possess themselves of one of Burt's solar compasses; for it is made their duty to be acquainted, before entering upon a survey, with the absolute variation of the needle, at the time and place where the survey is to be made, and to note the same upon their certificates, and no returns are either lawful or can be received as evidence in any court, without it. It may be supposed that this might be dispensed with, when it is known that the surveyor in the subdivision of a section, must be governed by the section and quarter section posts already established, and that the business of dividing a section, therefore, is merely intersecting these posts with his line, without the power to change them when wrong; and when smaller divisions are required, of taking equal distances between them, whether the full complement of acres be wanting or otherwise.

But though most surveyors have a meridian line for their own accommodation, generally in their immediate neighborhood, from which they can determine the magnetic changes, yet when their duties require them in opposite parts of a county, where the variation not unfrequently differs a degree, it is obvious that time must be devoted to an observation of some star to obtain the variation, if it be a clear night, or if otherwise, the survey must be suspended until a more favorable time. Besides all this, his pay is fixed at a stated price per mile, and that price barely a compensation. Now, either to make the business desirable, and at the same time to fulfill the conditions of the law, he must have a ready method of getting the variation, or must spend his time in so doing, without an equivalent for his labor; and while the law remains as it is, no instrument besides the solar compass can enable the surveyor, with profit, completely to fulfill the intent of the statute.

This question is of no less importance now, than it will be

*Part first, title second, chapter third, section sixty-nine, revised statutes. In all surveys made as aforesaid, the course shall be stated, according to the true meridian, and the variations of the magnetic meridian shall also be stated, with the day, month and year.

SEC. 70. The surveyor and his deputies may demand and receive for their services, the following fees, to wit: for each mile actually run with the compass, and measured with the chain, three dollars: Provided, That the necessary chainmen and markers be furnished by the surveyors or his deputy, at the request of the parties requiring the survey; but if the chainmen and markers are furnished by the party for whom the survey is made, or if the chainmen and markers be not necessary, then the surveyor and his deputies shall recive for each mile run, seventy-five cents, &c.

in future, when all traces of the original lines shall have been lost, which is the case already, wherever the county has become settled, and roads or fields have been opened along the boundaries of section lines.

AREA OF THE LOWER PENINSULA OF MICHIGAN-former maps and descriptions erroneous-definite information now obtained.

We have now the means of ascertaining, with the utmost degree of accuracy, the precise area of the lower peninsula, au accuracy characteristic of the plan pursued where the general government has the control of the surveys. These surveys afford the greatest facility in determining the boundaries and extent of every portion over which they have been made, from an eighty acre lot to a whole territory; and such has been their progrees within the last three years, that there remains only the small fraction of thirty-six townships unsub-divided; but these, being mostly in the interior, present no difficulty in determining at once the exact number of square miles contained within the boundaries under consideration. I have with extreme care, multiplied together every fraction, and find the whole amount to be 41,304 square miles or 26,434,560 square

acres.

In giving the above estimate, we cannot avoid the opportu nity of confronting its results with the compilations of draftsmen and geographers, who, in relation to the peninsula, have heretofore, in the main, copied such estimates as have fallen in their way, whether right or wrong. But little care has been exercised, even by those who ought to have been better acquainted with the errors which have always characterized not only the maps, but the descriptions of the peninsula. It is to be hoped that the period of such errors is now past, and that while most existing publications become obsolete, they will be replaced by correct ones; for it is not too much to say, that now, not only correct, but precise and definite information can be obtained, and wherever a dependence shall be placed upon former maps and descriptions, so far will their numerous and universal mistakes be the means of leading into important

errors.

ELEVATION AND DEPRESSION OF THE WATER IN THE GREAT LAKES-the maximum for 1838, 1839, and 1840-lowest stages of water of longer continuance than the higher stages— evaporation-semi-annual alternations-effects of winds—the apparent tides fortuitous-reaction of the waters-table of elevation and depression for 1840.

The last year is the second since the unusual elevation of the waters of the lakes; since which time there has been yearly

a remarkable coincidence in the ration of their subsidence, the more unlooked for, when taken in connection with the causes which tend to equalize the amount of falling water, in the form of rain, snow, and dew, with the constant action of evaporation.

In bodies of water like these lakes, slight changes in the seasons produce visible effects, inasmuch as they have no equalizing under-currents.

The quantity of rain must have been much less, and the evaporation more, than for many years past, to have produced the decrease mentioned below. This decrease amounts in the first year to one quarter of the total rise, and in the second to one-half, making the proportion each year as thirty-three to forty-four nearly.

The maximum of August, 1838, was five feet three inches above that of 1819; that of 1839, three feet eleven inches; and that of 1840, two feet seven and one-half inches. The ratio of decrease, therefore, between the highest water in 1838 and 1839, is one foot four inches; and between the highest water in 1839 and 1810, one foot three and one-half inches.

Its rate of decrease is much more rapid than that of its increase from 1819 to 1838. In 1830 it was only two feet above the level of 1818; in 1836, three feet eight inches; having risen one foot eight inches in six years. In 1837, it was four feet three inches, increase, seven inches; in June, 1838, five feet; increase, nine inches; and in August of the same year, five feet three inches. Having been nineteen years in attaining the maximum of five feet three inches, and only two years in reducing that height one-half, or to the average year of 1833. Thus the rapidity of its decrease in two years, equals the increase of five years.

I have not been able to ascertain whether the decrease of former years was thus sudden, or whether the period of the minimum, or lowest stage of water, continues for any great length of time; it is quite probable, however, that it does, and that the overflowing of the lands caused by the maximum rise, is but temporary, and only for one year, whence immediately commencing its decrease, it arrives very soon at its former standard, and remains there with little variation. Indeed, this is the more probable, from the example of the last three years, and from the appearance of long and undisturbed processes in the growth of trees and vegetation, with the formation of permanent channels in the interim, as well as the security felt by those who have erected buildings and planted orchards formerly, upon those lands which were inundated.

The diminution in a given quantity of water, exceeds by evaporation, all the supplies which it receives from rain, that is, the average amount of falling water, is equal per year to

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