Mr. C. Tomlinson's Experiments on the Electrical Fly . 205 



account of the points easily transmitting electricity) acquireth 

 a strong electricity analogous to that of the points ; and there- 

 fore the air and the points must repel each other.' 3 This expla- 

 nation, he adds, " is confirmed by observing that the above fly 

 not only does not move in vacuo, but even if placed under a close 

 receiver it will turn but for a little while and then stop ; for the 

 quantity of air contained in the receiver may become readily and 

 equally electrified"*. Cavallo states in a note, that when the 

 finger is placed outside the glass opposite to one of the points of 

 the fly, they will move again briskly, and by altering the position 

 of the finger round the glass its action may be continued till 

 most of that part of the glass is charged. He explains the action 

 of the finger in withdrawing from the outside of the glass a por- 

 tion of its natural electricity, whereby "it receives the fluid of 

 the electrified air on its inside surface ; hence this air is put in a 

 state of being again electrified by the point of the fly, which 

 renews the motion." 



Cuthbertson saysf that the motion of the fly " is occasioned 

 by the action of the electric fluid against the electrified air near 

 the point." 



Biot %, adopting the theory of Poisson, that electricity is held 

 to the surface by atmospheric pressure, states that the motion 

 of the fly could hardly be produced in an absolute vacuum, 

 because the electricity of the fly would be instantly dissipated ; 

 but that the motion can readily be produced in free air if the 

 points be sufficiently sharp to enable the electricity to accumulate 

 at their extremities so as to overcome the atmospheric pressure. 

 The motion is then produced by reaction precisely in the same 

 manner as if the wire were a hollow tube, bent at the ends in 

 opposite directions, from which water or mercury were flowing. 

 In such case the pressure on the orifices being reduced to zero, 

 while that on the tube opposite the orifice is still acting, motion 

 is produced in a direction opposite to that of the issuing fluid, 

 and the wire thus turns on its centre. 



The writer of the article u Electricity " in the Encyclojicedia 

 Metropolitana, published about 1824. says (p. 78), "Each of 

 these points will give off a current of electricity, which from the 

 reaction of the resisting medium (the air), will cause the system of 

 points to revolve backwards with considerable rapidity.-''' 



Roget§ thus explains the action of the fly: — "Each of the 



* A Complete Treatise on Electricity, third edit. (1786) vol. i. p. 296. 



t Practical Electricity (1807), p. 24. 



X Traite de Physique (1816), vol. ii. p. 324. 



§ Library of Useful Knowledge. Natural Philosophy, vol. ii. (1832), 

 "Electricity," p. 26. In my copy of this work the engraving of the fly 

 with its supporting stem is placed upside down. 



