VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 36 1 



preceding part a, comes quickly against the rubber ; the rubber being already 

 negative, and not being capable of losing that electricity very quickly, must in- 

 duce a stronger positive electricity in the part b, which is now opposite to it ; but 

 this part b cannot become more positively electrified, unless it receives the electric 

 fluid from some other body, and therefore some quantity of electric fluid passes 

 from the lowest part of the rubber to the part b of the glass, which additional 

 quantity of electric fluid is retained by the part b only while it remains in contact 

 with the rubber ; for after that, its capacity being diminished, the electric fluid 

 endeavours to escape from it. Thus we may conceive how every other part of 

 the glass acquires the electric fluid, &c. and what is said of the cylinder of an 

 electrical machine may, with proper changes, be applied to any other electric 

 and its rubber. 



It appears therefore, that according to this theory, a part of the rubber, viz. 

 that which the surface of the glass cylinder enters in turning round, must serve 

 to furnish the electric fluid to the glass, and the upper part must be possessed of 

 a negative electricity capable of inducing a positive electricity in the glass conti- 

 guous to it. In fact, this seems to be confirmed by the general practice and ex- 

 perience; for that rubber answers best for a common electrical machine, which can 

 easily conduct the electric fluid with its under part, and the upper part of which 

 is more ready to acquire, and to retain, the negative electricity; hence the rub- 

 bers are generally furnished with amalgam below, and with a piece of silk above; 

 hence also, if the cylinder of the machine be turned the contrary way, it will 

 produce little or no electricity. It often happens, that the part which conducts 

 the fluid, and that which acquires the electricity contrary to that of the electric, 

 are not so disposed in a rubber as is above described; but it remains always true, 

 that the rubber must be possessed of those two properties, viz. to conduct the 

 electric fluid very readily in one or more parts, and to acquire, as well as retain, 

 on other parts, an electricity contrary to that acquired by the electric that is to 

 be rubbed with it. 



//. The Croonian Lecture on Muscular Motion. By George Fordyce^ M, D., 



F.R.S. p. 23. 

 In considering muscular motion, I must begin with some observations on 

 motion in general, and with that well known and self-evident axiom, that one 

 particle of matter, considered by itself, will remain at rest if it be at rest, and 

 will continue in motion if it be in motion, and in the same direction. This has 

 been called the vis insita, or vis inertiae, of matter. It may be said in other 

 words, that a single particle of matter being at rest, would therefore always con^ 

 tinue at rest, if it were not for some external impulse made on it. This impulse 



VOL. XVI. 3 A 



