42 2 Of the Propagation of Heat 



In the middle of a glass body, of a pear-like form, 

 about 8 inches long, and i\ inches in its greatest di- 

 ameter, I suspended a small mercurial thermometer, 5I 

 inches long, by a fine thread of silk, in such a manner 

 that neither the bulb of the thermometer, nor its tube, 

 touched the containing glass body in any part. The 

 tube of the thermometer was graduated, and marked 

 with fine threads of silk of different colours, bound 

 round it, as in the thermometers belonging to the other 

 instruments already described , and the thermometer 

 was suspended in its place by means of a small steel 

 spring, to which the end of the thread of silk which held 

 the thermometer being attached, it (the spring) was 

 forced into a small globular protuberance or cavity, 

 blown in the upper extremity of the glass body, about 

 half an inch in diameter, where, the spring remaining, 

 the thermometer necessarily remained suspended in the 

 axis of the glass body. There was an opening at the bot- 

 tom of the glass body, through which the thermometer 

 was introduced; and a barometrical tube being soldered to 

 this opening, the inside of the glass body was voided of 

 air by means of mercury ; and this opening being after- 

 wards sealed hermetically, and the barometrical tube 

 being taken away, the thermometer was left suspended 

 in a Torricellian vacuum. 



In this instrument, as the inclosed thermometer did 

 not touch the containing glass body in any part, on the 

 contrary, being distant from its internal surface an inch 

 or more in every part, it is clear that whatever Heat 

 passed into or out of the thermometer must have passed 

 through the surrounding Torricellian vacuum ; for it can- 

 not be supposed that the fine thread of silk, by which 

 the thermometer was suspended, was capable of conduct- 



