1825.] Proceedings of Philosophical Societies. 143 



Article XI. 



Proceedings of Philosophical Societies. 



ROYAL SOCIETY. 



Dec. 23.— Two papers by the Rev. Baden Powell, MA. FRS. 

 were read, supplementary to a former communication on 

 Radiant Heat; and the Society adjomnied to 



Jan. 13, 1825; when John Bell, Esq. and William Scoresby, 

 Jun. Esq. were admitted Fellows of the Society; and A Descrip- 

 tion of a Floating Collimator, by Capt. H. Kater, FRS. was 

 read. 



This instrument is destined to supply the place of a level or 

 plumb line in astronomical observations, and to furnish a ready 

 and perfectly exact method of determining the position of the 

 horizontal or zenith point on the limb of a circle or zenith sector. 

 Its principle is the invariability with respect to the horizon of 

 the position assumed by any body of invariable figure and weight 

 floating on a fluid. It consists of a rectangular box containing 

 mercury, on which is floated a mass of cast iron, about twelve 

 inches loag, four broad, and half an inch thick, having two short 

 uprights, or Y's, of equal height, cast in one piece with the rest. 

 On these is firmly attached a small telescope furnished with 

 cross wires, or, what is better, crossed portions of the fine 

 balance spring of a watch, set flat-ways, and adjusted very 

 exact/)/ in the sidereal focus of its object glass. The float is 

 browned with nitric acid to prevent the adhesion of the mercury, 

 and is prevented from moving laterally by two smoothly polished 

 iron pins, projecting from its sides in the middle of its length, 

 which play freely in vertical grooves of polished iron in the sides 

 of the box. When this instrument is used, it is placed at a 

 short distance from the circle whose horizontal point is to be 

 ascertained on either side (suppose the north) of its centre ; and 

 the telescopes of the circle and of the collimator are so adjusted 

 as to look mutually at each other's cross wires (in the manner 

 lately practised by Messrs. Gauss and Bessel), first of all 

 coarsely by trial, applying the eye to the eye-glasses of the two 

 instruments alternately ; and finally by illuminating the cross 

 wires of the collimator with a lanthorn and oiled paper, taking care 

 to exclude false light by a black screen having an aperture equal 

 to that of the colHmator, and making the coincidence in the 

 manner of an astronomical observation, by the fine motion of the 

 circle. The microscopes on the limb are then read off", and thus 

 the apparent zenith distance of the coUimating point (intersec 

 tion of the wires) is found. The collimator is then transferred 



