Royal Society. 313 



Find the electromotive force of one cell of the battery " (p. 67). 

 It -will be well to add that in the problems the author has " almost 

 exclusively employed the nomenclature and the system of units 

 recommended by the Committee of the British Association."' 



XXXIX. Proceedings of Learned Societies. 



EOYAL SOCIETY. 



[Continued from p. 233.] 



March 23, 1876.— Dr. J. Dalton Hooker, C.B., President, in the 



Chair. 



rpHE following paper was read : — 



*■ " On the Nature of the Force producing the Motion of a Body 

 exposed to Bays of Heat and Light." By Arthur Schuster, Ph.D., 

 Demonstrator in the Physical Laboratory of Owens College. 



Mr. Crookes has lately drawn attention to the mechanical action 

 of a source of light on delicately suspended bodies in vacuo ; I have 

 made a few experiments which will, I think, throw some light on 

 the cause of the phenomenon, and assist us in the explanation of the 

 manifold and striking experiments made by Mr. Crookes. 



"Whenever we observe a force tending to drive a body in a cer- 

 tain direction, we are sure to find a force equal in amount acting in 

 the opposite direction on the body from which the force emanates. 

 It was with the view of finding the seat of this reaction that I have 

 made a few experiments. 



H the force is directly due to radiation, the reaction will be on the 

 radiating body; if, on the other hand, it is due to any interior action, 

 such as the one suggested by Prof. Beynolds, the reaction will be 

 on the enclosure of the moving bodies. I have been able to test this 

 by experiment, and I have found that the action and reaction is en- 

 tirely between the light bodies suspended in vacuo and the exhausted 

 vessel. 



The instrument best fitted for an experimental investigation of 

 this kind is the one which has been called " radiometer " by Mr. 

 Crookes. These instruments have been made in great perfection by 

 Dr. Geissler, of Bonn, under the name of " light-mills." Thanks 

 to the courtesy of Prof. Reynolds, I have been enabled to work 

 with such an instrument. The " light-mill " was suspended by means 

 of two cocoon fibres, forming a bifilar suspension, from the top of a 

 vessel which could be exhausted. A slight movement of the enclo- 

 sure could be easily detected by means of a concave mirror attached 

 to it. A beam of the oxyhydrogen lamp was concentrated on the 

 light-mill, which then revolved about 200 times a minute. 



The light was cut off at the beginning of the experiment by 

 means of a screen, and the position of rest of the glass vessel was 

 read off by meano of the dot of light on the scale. The screen was 

 then suddenly removed, and in every case a large deflection of the 

 glass vessel was observed. The vessel was deflected in the opposite 

 direction to that in which the mill turned. When the velocity of 



