VOL. LXIX.J PHILOSOPHICAL TRANSACTIONS. bgg 



contrived an apparatus with a double plate, by which all the electrical fire col- 

 lected by the 8 cushions was forced upon the conductor, so that none of it 

 could be thrown back, upon the centre, though made of brass. His contrivance 

 consisted in placing between the glass plates a strong glass ring, about 2 inches 

 in diameter, so that the brass centre passed through the middle of it. This ring 

 was stuck to the plates with sealing wax or some other non-conducting cement ; 

 and the space between the centre and the ring was carefully filled with the same 

 non-conducting substance. The conductor had 2 branches, each of which was 

 placed between the 2 glasses, reaching very near to the glass ring. By this me- 

 thod all, or almost all, the electricity excited by the 8 cushions, was forced to 

 pass upon the conductor, there being no way to reach the brass centre,, between 

 which and the conductor all communication was cutoff by the above mentioned 

 glass ring being filled up with a non-conducting cement. The power of such a 

 machine, notwithstanding the plates were not above 15 inches diameter, was 

 very astonishing. I saw one made in London with this improvement, the glass 

 plates being 18 inches diameter, by which a coated jar of 2 quarts was fully 

 charged in less than 5 seconds. 



Mr. C. Cuypers, an ingenious electrician at Delft, has not a little contributed 

 to the improvement of these machines, by making them less liable to be affected 

 by damp weather. This gentleman, considering that all glasses are not equally 

 fit for electricity, and that J. H. Wartz, and after him Professor Musschen- 

 broek, were of opinion, that glass, in the composition of which there enters a 

 great deal of alkaline salt, is very apt to attract moisture from the air, and 

 therefore less proper for electricity, which defect they thought might be cor- 

 rected by exposing it to a violent and continued heat, took a proper advantage of 

 this knowledge in the improvement of the machines with flat glasses. He found 

 that glasses which have been many years exposed to the warm air of a room, 

 very old looking-glasses for instance, become by this means much harder, so as 

 better to resist the force of a file ; and are then much better for electrical ma- 

 chines : and that such glasses become still incomparably better, if they are ex- 

 posed to a considerable degree of heat during some months : the heat forcing 

 out of the glass, at least out of its surface, the alkaline salt, not vitrified, which 

 is to be found upon it, and may be known by the taste. 



In December 1777 I saw one of these double-plated machines at Mr. Cuyper's 

 house, and found it do admirably well, though the weather was at the time very 

 damp, and the machine kept in a room in which there never was any fire made, 

 and though the cushions had no amalgama upon them : they were made of 

 yellow Turkish leather, stuffed with fine shavings of cork, rammed in them ; 

 and had been pressed to give them an equal smooth surface. The same gentle- 

 man found glasses, prepared in the above-mentioned way, far superior in strength 



