5O Inquiry concerning the Nature of Heat, 



A distinct and satisfactory idea may be formed of the 

 instrument I have been describing, from Fig. 2 (Plate 1 1.). 



AB is a board, 27 inches long, 9 inches wide, and i 

 inch thick, which serves as a support for the bent tube 

 CDE, at the two extremities of which the two balls are 

 fixed. The two projecting ends of the tube, C and E, 

 which are in a vertical position, are each 10 inches 

 long ; and the horizontal part D of the tube, which is 

 fastened down on the board, is 17 inches in length. 



The balls are each 1.625 inches in diameter. The 

 diameter of the tube is such, that i inch of it in length 

 would contain 15 grains Troy of mercury. 



The pillar F, which, by means of a horizontal arm 

 projecting from it, serves for supporting the circular 

 vertical screen represented in the figure, is firmly fixed 

 in the board AB. 



This circular screen (which is made of pasteboard, 

 covered on both sides with gilt paper) serves for pre- 

 venting one of the balls of the instrument from being 

 affected by the calorific rays proceeding from a hot body 

 which is presented to the opposite ball. 



Besides the circular screen represented in the figure, 

 several other screens are used in making experiments ; 

 for the instrument is so extremely sensible, that the 

 naked hand presented to one of the balls, at the dis- 

 tance of several inches, puts -the bubble in motion ; and 

 it is affected very sensibly by the rays which proceed 

 from the person who approaches it to make the experi- 

 ments, unless care be taken, by the interposition of 

 screens, to prevent those rays from falling on the balls. 

 These screens can be best and most readily made by 

 providing light wooden frames, about two feet square, 

 and half an inch in thickness, and covering them on 



