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Dr. Young's Refractions, the Barometer being at 30 inches, and the internal Thermometer at 50, or the external at 47, degrees; with the corrections for + one inch in the barometer, and for one degree in the thermometer of Fahren
heit. From page 19 of Vol. 1st of Pearson's Practical Astronomy.
The correction for an increase of altitude of one inch in the barometer, or for a depression of one degree in the thermometer, is to be added to the tabular refraction; but when the barometer is lower than 30 inches, or the thermometer higher than 47 degrees, the correction becomes subtractive.
When great accuracy is required, 0,003 inch should be deducted from the observed height of the barometer, for each degree that the thermometer near it is above 50 degrees, and the same quantity added for an equal depression.
Logarithm for converting Sidereal into Mean Solar Time +9.9988126. 66 Mean Solar into Sidereal Time +0.0011874.
A second of time, at the Equator, contains 1521 feet.
THE UNITED STATES NATIONAL OBSERVATORY AT
Lieut. M. F. Maury, A. M., Superintendent.
THIS institution was founded in 1842, under an act of Congress appropriating a sum of money to erect a Depot for the Charts and Instruments of the Navy of the United States, whence our public vessels might be suitably supplied with the nautical works requisite for their safe navigation. The site of the observatory is a beautiful one, commanding a fine view of Georgetown, Washington, Alexandria, Fort Washington (opposite Mount Vernon), and the Potomac River for several miles above and below the city. It occupies University Square, a plot of ground comprising seventeen acres, in the suburbs of Washington, on the left bank of the Potomac, and about one hundred feet above tide water. This was a reservation made by General Washington for the site of a great university. The observatory, however, is a Naval Institution, conducted entirely by naval officers, under the direction of Lieut. M. F. Maury, A.M., as principal superintendent. The law requires that the superintendent or director shall be either a Lieutenant, Commander, or Captain in the Navy of the United States.
The main building is of brick, fifty feet square, and two stories in height, surmounted by a revolving dome of twenty feet diameter in the clear, in which is placed the large equatorial, a splendid instrument, from the manufactory of Merz & Mahler, Munich. Attached to the main building, as observing rooms, are wings extending eighteen feet to the east and west, and a projection of thirty-six feet (in two apartments) to the south. The observatory is furnished with a set of excellent astronomical instruments, consisting, 1, Of the large refractor, in the dome already alluded to, of 14 feet focal length, with an object glass having 9 inches clear aperture; it is equatorially mounted, and furnished with clock work. 2. A transit instrument of 7.1 feet focal length, and 5.3 inches clear aperture; made by Ertel & Son, Munich, and mounted on the meridian in the west wing, where there is also a clock with a mercurial pendulum, made by Parkinson & Frodsham. In the east wing is, 3. a meridian circle, by the same makers; its object glass having 3.8 inches aperture, with a focal distance of 4.9 feet. This instrument is provided with a 30 inch circle, divided into arcs of 3', and read to seconds and tenths by four microscopes. A few feet from this, in the same room, stands, 4. the elegant mural circle, an English instrument, by Troughton & Sims, of 5 feet diameter, divided into arcs of 5' value, and furnished with six reading microscopes, with which subdivisions of the circle are obtained in seconds and parts of seconds. The object glass of the telescope is of the same size with that of the meridian circle, with a focal length of 5 feet; the clock has a mercurial pendulum, and was made by Charles Frodsham.
In the south wing, first apartment, is, 5. the fine transit instrument made by Pistor & Martins, of Berlin, for the prime vertical. It is mounted at one end of its axis, and outside of its supports. It is reversed from one to the other side of these twice during every observation ; and though it weighs upwards of 1,000 pounds, so perfect is its system of coun rpoises and the reversing apparatus, that a child can lift it from its Ys, reverse and replace it in them in less than one minnte. The focal length of this telescope is 6.5 feet, with a clear aperture of 4.9 inches; the clock has a gridiron pendulum, and is of Charles Frodsham's make.
The observations made with instruments thus mounted, are among the most accurate known in astronomy; and those now making in Washington have not been surpassed by any at the oldest observatories in Europe. The accordance of the resulting declinations is such that it is believed that there is not a single one made during the whole of the last year, which differs as much as 1" from the mean of all the others upon the same star. A catalogue of all the stars within the reach of this instrument has been already commenced at the national observatory, which, when completed, will be regarded by astronomers as a standard work; and perhaps as the most accurate catalogue that has ever appeared. A careful revision of this catalogue, in after years, will probably lead to highly valuable and interesting results. But wonderful as is the degree of accuracy in the results obtained from this instrument, Lieut. Maury has discovered imperfections in it which he has sought to correct by another. For this purpose he furnished Messrs. Ertel & Son with plans and drawings of an improved instrument, which has been lately received at the observatory; and which we understand those skillful makers pronounce to be the most complete astronomical instrument that they have ever made. It is intended to be mounted temporarily in the prime vertical in the other apartment of the south wing. But after it has been applied in this direction to the investigation of the several problems which are connected with its position cast and west, it is then to be turned permanently on the meridian, where it will be used for observations upon atmospherical refractions, parallax, etc., and for the purpose of determining both right ascensions and declinations, since it combines all the capacities both of the meridian transit instrument and the mural circle. But as it is the first instrument ever procured in this country for the purpose of investigating the subject of atmospherical refractions, Lieut. Maury has called it the "Refraction Circle," though its objects and uses are by no means confined to this subject alone. For instance, in the meridian it is both a mural and a transit instrument, and by reason of its facilities for reversal, a zenith sector; also, it is well adapted for measuring the difference, in zenith distance, between north and south stars. In the prime vertical it becomes an improved zenith sector, and takes the place of the prime vertical transit instrument, with all the advantages superadded of an altitude instrument in that direction.
We have not been able to obtain accurate measurements of all parts of this instrument. The telescope is 83 feet long, with a clear aperture of 7 inches. It is supported in the middle of the axis, between two piers; it has two 4 feet circles, one on each end of the axis, divided on gold into arcs of 2' value. Each circle is provided with six reading microscopes. The telescope has two micrometers, one moving in azimuth, the other in altitude. It is so contrived that the wires, and not the field, are illuminated; and every eye-piece, even of the highest powers, just as it is used, and without alteration of any kind, becomes a collimating eye-piece, by simply turning the telescope down upon a basin of mercury. At one end of the axis, and concentric with it, are two cross lines situated in the focus of a lens placed at the other end of the axis. Upon the prolongation of this axial line at either end, and attached to the same piers which support the telescope, are placed two collimators, which are ingeniously contrived to stand in the place both of the striding level and meridian marks, and which are to be used for determining the level of the instrument, the figure of the axis, and the eccentricity of the pivots. There is a further provision for detecting the slightest changes in the deviation of the level of the axis from a normal state, on account of unequal expansion of the supporting piers, which are of granite. This consists of another beautiful contrivance by means of a couple of invariable rods, or rods of well-determined expansibility, which are inserted vertically through the piers; and which, by turning a mirror, show the minutest alterations which can take place from this source, in the level or inclination of the axis of the instrument. Thus the imperfections of the spirit level are in a great measure avoided; and all the advantages, with but a part of the inconveniences, of a striding level to reverse with the instrument, are secured.
Appreciating the advantages which these fine instruments afford, the superintendent appears to have begun with the determination of so employing them, that their results, while they should be the most useful, would at the same time most redound to the credit of the observatory and the navy, and to the honor of the country. The national observatory being a naval institution, its first object and duty would seem to be to undertake those observations which are most immediately connected with navigation, and the results of which might afford the data requisite for calculating an American Nautical Almanac. Accordingly the naval astronomer at once commenced with a regular and systematic series of observations upon the sun and moon, the planets, and a list of fundamental stars, comprising those of the greatest magnitudes and of the most favorable positions, to be used as the standard stars in the almanac. But these did not give full employment either to the instruments, the observers, or the computers. Accordingly, after having adjusted his plans, Lieut. Maury took an early opportunity, in 1845, of making them known to the Secretary of the Navy, and of obtaining from him authority to undertake observations for a most extensive