PHILOSOPHICAL 
TRANSACTIONS. 
XI. On a new form of the differential thermometer, with some oj 
its applications. By William Ritchie, A. M. Rector of 
Tain Academy. Communicated by J. F. W. Herschel, Esq. 
Sec. R. S. 
Read December 21, 1826. 
I n using metallic reflectors for experiments on radiant heat, 
the results are liable to considerable uncertainty, in conse- 
quence of the imperfection of the reflectors, and of the 
difficulty of placing the bulb of the thermometer exactly in 
the focus, when the source of heat is removed to different 
distances. The following contrivance is free from these 
objections, and will illustrate the various properties of radiant 
heat in a more simple and accurate manner than by the more 
expensive and imposing method of reflectors. 
The instrument consists of two cylindrical chambers of 
very thin brass, or tin plate, from two to six or eight inches 
in diameter, and from a quarter of an inch to an inch thick. 
These chambers, like those of the photometer formerly 
described, are connected by a thermometer-tube bent in the 
form of the letter U, having small bulbs blown near its 
upper extremities. The tube, like that of the differential 
S 
MDCGCXXVII. 
