HOW THE SUN WARMS THE EARTH — ABBOT 



161 



when they are fully absorbed upon a black surface. Hence we use 

 sensitive blackened electrical thermometers rather than the eye, the 

 photograph, or even the photoelectric cell when we w^ish to compare 



riG. JI 



Jl 



^ 



\\\\K?\\V\\\\\\\\\\\\\\\\^^^^ 



FiGCRH 5. — Vacuum Bolometer. 



Sensitive strips h h', mounted on the copper bloelts c d' form two arms of a Wheat- 

 stone bridge completed by the two colls b b'. Solar spectral rays are admitted by a 

 vestibule with diaphragms and may be adjusted by the eyepiece. The vessel, a, is highly 

 evacuated. 



accurately the energy of one ray with the energy of another ray 

 of a different color or wave length. My predecessor, Secretary 

 Langley, invented the bolometer (fig. 5), a delicate electrical re- 



