VOL. LXIX.3 PHILOSOPHICAL TRANSACTIONS. 571 



ness of common air may even be useful to posterity, in order to ascertain whether 

 our atmosphere degenerates in a length of time. This curious inquiry, together 

 with the method, &c. are the production of the 1 8th century ; and our de- 

 scendants must have some gratitude for the philosophers who found out, as well 

 as for those who improved it. If our ancestors had known and transmitted it to 

 us, we should perhaps at present be able to judge of one of the greatest changes 

 of our globe, of a change which very nearly interests human life. 



XXIX. Of some Experiments in Electricity. By Mr, Wm. Sivift. p. 454. 



One particular addition Mr. S. had made to his apparatus, consists in what he 

 calls an anti-conductor: it is exactly like the prime conductor; but is fixed to the 

 cushion of the machine, and consequently, when the cylinder is put in motion, 

 the anti-conductor is charged negatively, that is, the electric matter is diminished 

 in it, in the same proportion as it is increased in the prime conductor in the same 

 time. Another thing peculiar to this machine is, that the whole is insulated; so 

 that being able to collect the electric matter without any connection with the 

 earth, and having at the same time bodies or conductors positively and negatively 

 electrified, he was enabled by this apparatus to exhibit many experiments more 

 analogous to the natural effects of lightning from the clouds, than it is possible to 

 do with only one conductor positively electrified; because in nature clouds are 

 constantly flying in the air which are differently electrified, and, discharging 

 themselves into each other, produce the lightning often seen in the atmosphere. 



It may be proper to mention a few common experiments and observations, to 

 show that the 2 conductors are differently charged, that is, the one positively or 

 plus, the other negatively or minus, as soon as the cylinder is put in motion by 

 turning the wheel. 



1. When the cylinder moves, and a body approaches the prime conductor, 

 such body will draw a spark from that conductor at the same distance, and con- 

 sequently of the same length, as will be drawn from the approaching body by the 

 anti-conductor. And a pith ball is equally attracted by both. Which sufficiently 

 shows that both conductors are charged or electrified. 



2. The following common experiments will show, that they are differently 

 electrified. Take a wire with a small piece of cocoa wood, about J 4- inch long, 

 pointed, fastened to one end of the wire, and connecting the other end to the 

 anti-conductor; as soon as a conducting body approaches it, there is a bright 

 spark resembling a star, which appears to settle on the end or point of the wood; 

 but when the wire is connected with the prime conductor, there issues from the 

 end of the wood a pencil of rays diverging to the point towards the approaching 

 body; which, Mr. S. apprehends, demonstrates the conductors to be differently 

 electrified. 



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