Reflections on Heat. 177 



objects, each on the bulb to which it is presented, is of 

 precisely the same amount; hence we can calculate the 

 relative intensity of the radiation of each one of the 

 two objects from the extent of the surface presented 

 to the bulb, and from the square of its distance from 

 the bulb. 



If we desire to compare the calorific action of a warm 

 body with the frigorific action of a cold body, we begin 

 by protecting one of the bulbs of the instrument by the 

 screens, and then present to the other bulb the two 

 objects, regulating their respective distances in such a 

 manner that their actions exerted at the same time pro- 

 duce equal effects, that is, so that one warms the bulb 

 as much as the other cools it. 



The equality in the amount of action is denoted by 

 the remaining at rest of the bubble of spirit of wine 

 which serves as the index of the instrument, and when 

 this equality is established, the relative intensity of the 

 radiation from the objects in question is calculated from 

 the amount of surface which they respectively present to 

 the bulb, and from the squares of their distances from it. 



The sensibility of this instrument is so great that, 

 when it is at a temperature of 15 or 16 of Reaumur's 

 scale, if the hand be presented to one of the bulbs at a 

 distance of three feet, the heat radiating from the hand 

 is sufficient to cause the bubble of spirit of wine to move 

 forward several lines ; and the cooling influence of a 

 blackened metallic disk four inches in diameter, at the 

 temperature of melting ice, is such that, when presented 

 to the bulb at a distance of 1 8 inches, it causes the bub- 

 ble to advance in the opposite direction with a rapidity 

 which is very perceptible to the eye. 



By means of this instrument I have discovered, 



VOL. II. 12 



