52 Inquiry concerning the Nature of Heaf, 



part, the top of the box is covered by a sliding board, 

 2 feet 3 inches long, which passes in deep grooves, made 

 to receive it, in the sides of the box. A rack is fixed 

 to the under side of each of these sliding boards ; and 

 there is a small cog wheel in the box, the axis of which 

 passes through the sides of the box, and is furnished 

 with a winch in the front of the box. By turning round 

 these wheels by means of their winches (both of which 

 can be managed by the same person, at the same time), 

 the sliders may be moved backwards and forwards at 

 pleasure. 



In order to ascertain with facility and dispatch the 

 distances of the hot bodies from their respective balls, 

 the top of the front side of the wooden box is divided 

 into inches on each side of the fixed part of the cover 

 of the box ; and there is a nonius belonging to each of 

 the sliders, which is placed in such a manner as to indi- 

 cate, at all times, the exact distance of the hot body 

 from its corresponding ball. 



The level of the upper surface of that part of the 

 cover which is fixed is about \ of an inch higher than 

 the level of the upper surface of the sliders, in order 

 that, when a thermoscope longer than this fixed part is 

 placed on it, the sliders may pass freely under its two 

 projecting ends without deranging it. 



It is evident, from this description, that by placing 

 the thermoscope on the fixed part of the cover of the 

 box, with its two balls in a line parallel to the axis of 

 the box, and by placing the two hot bodies presented 

 to the two balls of the instrument (elevated to a proper 

 height) on stands set down on the sliders, an observer, 

 by taking the two winches in his hands, keeping his 

 eye fixed on the bubble, may, with the greatest facility, 



