Description of the Kew Observatory. 141 



South Hall. In the passage adjoining the south hall is placed the 

 standard clock, by French (9), the temperature of this part of the 

 Observatory being very constant. The greater part of the south 

 hall is taken up by a double-walled wooden structure in which various 

 observers have at various epochs swung pendulums to determine 

 gravity (see 'Roy. Soc. Proc.', vol. 14, pp. 425439 ; vol. 17, pp. 488 

 499; and 'Phil. Trans.', vol 181, pp. 537558). 



The wooden structure is also used for chronometer trials ; its tem- 

 perature can be raised and kept steady at 100F. by means of a 

 copper stove. 



Sextant Room. This contains an apparatus (3) for testing the 

 accuracy of graduation in the arc of sextants, which was designed 

 by Mr. T. Oooke ('Boy. Soc. Proc.', vol. 16, pp. 2-6). It consists 

 essentially of five collimating telescopes, with gas jets behind them, 

 which illuminate objects, in the shape of crosses, situated at the 

 principal foci. The collimators are fixed on a slate slab, carried by 

 brickwork. The emergent beams of light converge to a common 

 centre, and are inclined at certain known angles. The readings 

 given by a sextant for the magnitude of these angles supply the 

 data for determining the error at various points in its arc. 



FIRST FLOOR. 



North Hall. This is an octagonal-shaped room, surrounded by 

 glass cupboards, most of which contain instruments no longer in use. 

 Some of these are of considerable historic interest, such as Ronalds' 

 apparatus for examining atmospheric electric potential, Wheatstone's 

 galvanometer, Lord Kelvin's early form of mirror electrometer, and 

 various old patterns of magnetic instruments. 



Barometer Room. The standard barometers (4) were mounted by 

 Welsh, in 1855 ('Phil. Trans.' 1856, p. 507). The barometers 

 are read by means of a cathetometer (5), constructed by Mnnro. 

 Their cisterns are at a height of 34 ft. above mean sea level. The 

 room also contains the ordinary working standard barometer (6), 

 made by Newman ; this is compared from time to time with the 

 Welsh standards, and is used to standardise the barograph, obser- 

 vations being made with it at 10 A.M., noon, 2 P.M., 4 P.M., and 

 10 P.M. There are also receivers (7) and (7'), in which ordinary 

 mercury barometers and aneroid barometers respectively can be com- 

 pared at low pressures. 



A standard clock (8), by Dent, is used in the tests of watches 

 commenced in 1884 ; it is electrically connected with a chronograph 

 (10), which daily receives the Greenwich time signal. 



The room likewise contains a balance by Oertling, and a dividing 

 engine by Perreaux, similar to that employed by Regnault (' Roy. 



