226 Historical Review of Experiments 



precisely alike, four and a half inches in diameter, three 

 inches high, and open above. In each of these boxes, 

 an inch and a quarter from the bottom, I put a cir- 

 cular metallic disk, a quarter of a line in thickness, and 

 of the same diameter as the inside of the box. This 

 disk, which formed a sort of optical screen in the in- 

 side of the box, was fastened in its place by a number 

 of very short wooden pegs, which went through the side 

 of the box. 



In the middle of the bottom of the box was a circular 

 aperture, three quarters of an inch in diameter, closed 

 by a cork stopper. 



In this stopper was a hole of three lines' diameter, 

 into which fitted a small mercurial thermometer pro- 

 vided with an oval reservoir. The divisions of the 

 scale were engraved upon the tube itself.* By means 

 of this stopper the thermometer was introduced into 

 the inside of the box in such a way that its bulb was 

 situated in the axis of the box, and in the middle of 

 the space between the bottom and the metallic disk. 

 This space, which was designed to serve as a reser- 

 voir of heat, was filled with a certain quantity of flat 

 silver threads, which had been picked out of old silver 

 lace. 



In one box the metallic disk or reflector was brass; 

 in the second, tinned iron ; and in the third, ordinary 

 sheet-iron. 



* I had four such thermometers made for me in England, and they did me good 

 service throughout the whole course of my experiments on the subject of heat. Their 

 tubes were made of very hard glass, three lines in diameter, and were polished down 

 on one side so as to present a flat surface, on which the divisions of the scale were 

 etched with fluoric acid. The tubes are six or seven inches long, and the bulbs for the 

 mercury are pear-shaped, and consequently not so liable to get broken as cylindrical 

 ones. In the pointed or lower part of the pear, the glass can be quite thick without 

 any disadvantage. 



