350 Mr Ritchie m a neiv form of Differential Thermometer. 



the descent of the fluid. In one experiment the efFect was 

 IS** with the screen close to the instrument, and yet this fluid 

 rose only one degree when the screen was removed five inches 

 towards the heated ball. 



Hence in the preceding experiment the effect was not pro- 

 duced by heat radiating from the posterior surface of the 

 screen, but by heat actually radiating through the screen, in 

 the same manner as light radiates through water, or other 

 transparent fluids. 



Art. XXX.-— Ow a new form of the Differential Thermo- 

 meter^ with some of its applications. * By William 

 Ritchie, A. M. Rector of Tain Academy. 



Ok account of the difficulty of placing the bulb of the ther- 

 mometer exactly in the focus of a metallic reflector, when the 

 source of heat is removed to different distances, Mr Ritchie 

 has proposed the following instrument for experiments on ra- 

 diant heat. 



It consists of two cylindrical chambers of very thin brass or 

 tin plate from two to siic or eight inches in diameter, and from a 

 quarter of an inch to an inch thick. These chambers, like those 

 of the photometer, described in this Journal, Number iv. p. 340, 

 are connected by a thermometer tube bent in the form of the 

 letter U, having small bulbs blown near its upper extremities. 

 The tube contains a coloured fluid, such as sulphuric acid 

 tinged with carmine, alcohol, &c. The outer faces of the cy- 

 lindrical chambers are coated with lamp black, in order to ab- 

 sorb the radiant heat, which is quickly conducted to the inte- 

 rior of the chamber, and expands the included air. The scale 

 may be divided into any number of equal parts, as the results 

 are only to be compared with each other. 



Mr Ritchie next proceeds to determine by this instrument 

 the law of decrease of radiant heat with the distance. 



Exp. 1. A cylindrical vessel of tin plate filled with hot 

 water, and having its diameter the same as that of the cham- 



• This is a full abstract of Mr Ritchie's paper read before the Royal 

 Society, December 21, 1826. 



