Mr. C. Torulinson's Experiments on the Electrical Fly, 207 



causes the movement by a true recoil, analogous to that produced 

 by liquids or gases upon reacting machines during their escape." 



Pouillet* refers the motion to a recoil exactly similar to that 

 produced in the hydraulic tourniquet. 



Ganotf denies the truth of this statement, and falls back 

 upon the explanation given by the electricians of the last cen- 

 tury. " This motion/' he says, " is not an effect of reaction 

 comparable to that of the hydraulic tourniquet, as many physi- 

 cists suppose, but it is an effect of repulsion between the electricity 

 of the points and that which they communicate to the air. The 

 electric fluid, accumulating towards the point, escapes into the 

 air, which thus becomes charged with the same electricity as that 

 of the points ; it thus repels the points, and is at the same time 

 repelled by them. It is admitted that the tourniquet does not 

 rotate in vacuum ; and if we approach the hand near it whilst it 

 is turning in air, a slight wind will be felt, occasioned by the 

 displacement of the electrified air." 



Eisenlohr % also refers the motion to the repulsion occasioned 

 by the electrization of similar particles. 



In the foregoing statements we have no less than five theories 

 to explain the action of the electrical fly. First, there is what 

 may be called the recoil theory, advocated by Hamilton, Priestley, 

 the Encyclopedia Metropolitana, Despretz, Lardner, Pouillet, and 

 Harris. Secondly, the attraction theory, advocated by Kinners- 

 ley. Thirdly, the expansion theory, advocated by Beccaria. 

 Fourthly, the repulsion theory, advanced by Cavallo, Cuthbert- 

 son, Biess, Ganot, and Eisenlohr. Fifthly, the atmospheric 

 pressure theory of Biot, Boget, Becquerel, and De la Bive. 



Among these five sets of theories, the repulsion theory seems 

 to me best adapted to represent the truth ; but there are so 

 many interesting points of electrical science involved in the 

 behaviour of the fly under various circumstances, that it seeni3 

 difficult to represent its action in a few words. There is, first, 

 the behaviour of the fly in air; secondly, when wholly or par- 

 tially enclosed in vessels of glass or of metal; thirdly, when 

 placed in rarefied air of different pressures ; fourthly, when 

 poised in liquid dielectrics; and fifthly, when the points are 

 concealed or otherwise modified. 



In any theory of the fly, it seemed necessary from the begin- 

 ning of the inquiry to understand clearly the action of points ; 

 and desiring no better guide than Dr. Faraday § in this matter, 



* Elemens de Physique, seventh edit. (1856) vol. i. p. 508. 



t Traite de Physique, seventh edit. (1857) p. 566. 



t Lehrbuch der Physik (1860), p. 504. 



§ Experimental Researches in Electricity, vol. i. second edit. 1849. See 

 the whole of the instructive section entitled " The Electrical Brush," com- 

 mencing p. 454. 



