ynly 1 6, 1S74] 



NA TURE 



207 



Arbor. The object-glass of the equatorial has an aper^ 

 ture of 962 in and a focal length of 14ft. 4'S in. Its 

 powers of positive eye-pieces for use with its filar micro- 

 meter vary from 90 to 899. 



Descending from the dome, and passing the superin- 

 tendent's office, in which are a most excellent mean-time 

 clock, with others, in the electric circuit with the clocks 

 at the departments, ticking each, beat for beat, the 

 visitor finds himself in the libntry, now embracing 

 nearly 6,000 volumes. These are mostly works of the 

 highest standard value, astronomical and meteorological 

 observations and discussions, some being as old as the 

 year 1482, others representing the full work of the 

 European observatories and learned Societies to the pre- 

 sent date. 



From the library we pass into the transit-circle room, 

 built in 1S69, to admire the beautiful instrument, with its 

 collimators and its chronograph. The focal length of the 

 object-glass is 12 ft. i in. ; its clear aperture 8'52in. ; and 

 the power of its eye-pieces 135 to 396. The diameter of 

 its circles at the outer edge is 45-30 in., and at the gradua- 

 tion 43-40 in., both circles being divided to every two 

 minutes. The power of the reading microscopes is 45-3 

 diameters. Its collimators have a focal length of 2 ft. 

 II in. This instrument, under Profs. Newcomb, Hark- 

 ness, and Eastman, and their assistants, has had for its 

 chief work the more accurate determination of the stars 

 whose places are computed in the Nautical 'Almanac, 

 and of those needed by the Coast Survey. The chrono- 

 graph, made by Alvan Clark, is of the form known as the 





Flc. 6— The Unilccl Slates Naval Observatory. 



Vertical. E, Computer's Room for Great Equatorial F, Great Eqi 

 inlendcnt's Dwelling. I., Chronometer-Room. D, Door. \v, Windov 



Hipp chronograph, with modifications by Prof. Hark- 



ncss. 



Passing to the eastern wing there are seen, side by side, 

 the mural circle and the smaller transit instrument, with 

 their clock and chronograph. The mural circle has an 

 object-glass of 4-10 in., and a focal length of 5 ft. 38 i,T., 

 the highest power of the eye-pieces being 240. The 

 diameter of the circle at its outer edge, where the gradua- 

 tion is placed, is 60-35 '"• ^^ 's divided to every five 

 minutes; the power of its reading microscope is 17-1 

 diameters. The transit has a foc.tl length of 7 ft. 0-4 in., 

 and its object-glass an aperture of 5-33 in. 



The chronometer-room shows another and a distinct 

 but important office of the observatory. The relation of 

 all its work to the interests of practical navigation is 

 sufficiently clear. More than 200 time-keepers have been 

 at one time under care in this room. As many as eighty 

 in 1867 were condemned and withdrawn from use. It is 



as gratifying as it is creditable to American skill to find 

 that the chronometers of Messrs. Negus and Co., of Ne-w 

 York, equal, if they do not excel, any of foreign workman- 

 ship. 



From this room of the observatory the exact time is fur- 

 nished daily at 12 M. to the Western Union Telegraph Office 

 in Washington for dispatch throughout the United States. 

 The naval officer, standing by the standard mean clock, 

 and having the astronomical correction of that clock also 

 before him, at three minutes before 12 M. calls the tele- 

 graph operator at his office, and, at the instant of 

 noon, taps the electric key, giving the time to the com- 

 pany's office. He also drops the dome ball. The chro- 

 nometer-room is under the very efficient direction of 

 Commander A. W. Johnson, U.S.N. 



The seventeen annual volumes of astronomical and 

 meteorological observatioiis now published best set forth 

 in themselves the work of the obscrvato:y. The latest of 



