and the Mode of its Communication. 47 



as 5654 to 10,000, very nearly ; for the velocities are as 

 the times of cooling, inversely. 



Again, in the experiment No. 6, the sides of the in- 

 strument No. 2 being covered with four coatings of 

 spirit varnish, the instrument was found to cool through 

 the given interval of 10 degrees in 30^ minutes. 



In that time, a quantity of heat = 1627 parts, must 

 have made its way through the covered ends of the in- 

 strument ; and the remainder, = 8373 parts, must have 

 made its way through its varnished sides. 



This quantity, 8373 parts, would have required 

 66\ minutes to have made its way through the naked 

 sides of the instrument; and, as it actually made its 

 way through the varnished sides of the instrument in 

 303- minutes, it appears that the velocity with which 

 the heat was given off from the naked metallic surface, 

 was to the velocity with which it was given off from 

 the same surface covered with four coatings of spirit 

 varnish, as 66^ to 30^, or as 10,000 to 4566. 



Without pursuing these computations any farther at 

 present, and without stopping to make any remarks on 

 the curious facts they present to us, I shall hasten to 

 experiments, from the results of which we shall obtain 

 more satisfactory information. But, before I proceed 

 any farther, I must give an account of an instrument I 

 contrived for measuring, or rather for discovering, those 

 very small changes of temperature in bodies, which are 

 occasioned by the radiations of other neighbouring 

 bodies, which happen to be at a higher or at a lower 

 temperature. 



This instrument, which I shall take the liberty to 

 call a thermoscope, is very simple in its construction. 

 Like the hygrometer of Mr. Leslie (as he has chosen 



