on the Subject of Heat. 231 



only one that I could ever devise for such experiments 

 without fearing important objections on account of the 

 apparatus employed. 



I possess, also, various other thermometers, intended 

 simply to receive and collect within themselves the calo- 

 rific or frigorific rays which fall upon their surfaces. 

 The reservoir of each consists of two cones of very thin 

 sheet brass, which lie one within the other, and are 

 fastened to each other, on the under side, in such a way 

 that there is an empty space, not quite a line in width, 

 between the inner surface of the outer cone and the 

 outer surface of the inner cone. The inner cone is four 

 inches in diameter, four inches high, and ends in a 

 point above. The diameter of the outer cone is four 

 and a quarter inches, and it ends above in a cylindrical 

 tube three quarters of an inch in diameter and four 

 inches long. In this cylinder is fixed a glass ther- 

 mometer tube, and if the space between the two cones 

 be filled with linseed oil or coloured spirit of wine, this 

 instrument answers the same purpose as an ordinary 

 thermometer. The scale of this thermometer is fastened 

 firmly to this glass tube. 



The outer wall of the instrument is shielded and 

 protected from the calorific or frigorific influences of 

 the surrounding air by means of a cylindrical box of 

 dry wood, thickly coated with varnish, and filled with 

 eider-down ; into this the body of the instrument fits. 

 This cover is four and a half inches high, and is, on the 

 inside, of the same diameter as the lower part of the 

 outer cone ; and the tube of the instrument, with its 

 attached scale, goes through a hole made in the bottom 

 of the box. 



If, now, the outer blackened surface of the inner 



