2 



The Mechanical Action of Radiation. 



pendiciilar position, were attracted b}^ a moderate degree 

 of liglit, heat or electricity, but were repelled by the more 

 powerful impulses iQip^irted by the concentration of any of 

 these bodies." 



After the lapse of twenty years the phenomenon seems 

 to have been rediscovered by Mr. Mitchel. A description 

 of his experiment may be found in the first chapter of 

 Dick's Practical Astronomer (see also Scientific American^ 

 vol. XXXIII, 9) and reads very much as follows : 



A plate of very thin copper, an inch square, was fixed 

 upon the end of a fine wire ten inches long. A very deli- 

 cate magnet was fastened to the middle of the wire and 

 the whole, balanced on a pivot, w^as enclosed in a glass 

 case. The rays of the sun, collected by a concave mirror 

 of two feet diameter were thrown to a focus on the copper 

 plate. The plate began to move under the influence of 

 the condensed sunbeam and in about two seconds it had 

 traversed as many inches and struck against the side of 

 the box. This experiment was made with a view to prove 

 that " Light, though exceedingly minute, has a certain 

 degree of force momentum." 



Many years later, it was in 1863, the energy of radiation 

 seems to have revealed itself anew to the eminent Prof. 

 Joule. " By means of a cylindrical glass vessel, divided in 

 a vertical direction by a blackened [)asteboard diaphragm, 

 which extended to within one inch of the cover and of the 

 bottom of the vessel, and in the upper of which spaces was 

 delicately suspeiided a magnetized sewing needle furnished 

 with a glass index, he was able to detect the heat from a 

 pint of water heated to 30° C, placed in a pan at nine feet 

 distance; also that of a moonbeam admitted through an 

 opening in a shutter as it passed across his apparatus." 

 This description is extracted from a lecture by the Earl of 

 Rosse on the heat of the moon, given at the Royal Institu- 

 tion in May, 1873. 



