The Mechanical Action of Radiation. 



3 



In April, 1873, ignorant of all these earlier observations, 

 I read, in this room, a paper on " Convection applied to the 

 Detection of Heat,'" showing that a delicately suspended 

 needle would move in obedience to slight changes of tem- 

 perature in any body brouglit into its vicinity, and describ- 

 ing a Thermoscope^ quite sensitive, acting on this principle 

 (Journal Frank. Inst., vol. lxvi, 343). Further experiment 

 soon afterward resulted in the construction of an instru- 

 ment so sensitive that the needle would swing in response 

 to the heat radiated from the face of a person sitting at a 

 distance of thirty feet {Jour. Frank. Inst., vol. lxvii, 408). 

 The form of instrument finally adopted, and used also in 

 experiments to be described in the sequel, may be briefly 

 described as follows. In a chamber whose walls are, to a 

 considerabhi degree, impervious to heat, a glass thread, very 

 long and very light, is suspended horizontally by means 

 of two parallel silk fibres eighteen inches in length. One 

 end of this slender needle carries a small vertical disk of 

 paper. The small end of a conical metallic reflector passes 

 through the wall of the chamber and its opening is covered 

 with a piece of thin partially charred paper. The radia- 

 tions from any distant source of heat are concentrated 

 upon this scorched paper by the reflector. The needle- 

 disk is on a level w^ith the lower part of this paper and 

 moves toward or from it under the influence of every 

 change of temperature it experiences, approaching if it be 

 warmed and receding if it be very gently cooled, but ap- 

 proaching when the redaction of temperature is conside- 

 rable. 



In the meantime Dr. William Crookes, " while weighing 

 heavy pieces of glass apparatus in a chemical balance en- 

 closed in an iron case from which air could be exhausted," 

 noticed that ''when the substance w^eighed was of a tem- 

 perature higher than that of the surrounding air and the 

 weights, there appeared to be a variation of the force of 



