600 PHILOSOPHICAL TRANSACTIONS. [aNNO 1779. 



to a cake of resin used in the electrophorus. He published his method in a 

 pamphlet, Expose d'une Methode, &c. at the Hague, in 1778. 



Those who make use of plate machines, should carefully avoid putting the 

 apparatus near the fire, for the purpose of drying or warming it ; because the 

 sudden expansion of the glass by the heat cannot so quickly propagate itself 

 through its whole extent ; for the centre being commonly squeezed between 2 

 flat shives of brass, with a piece of leather between the metal and the glass, 

 does not acquire a similar degree of heat at the same time as the rest, and cannot 

 so easily expand ; and therefore the plate is in great danger of breaking. If, in 

 consequence of such a blunder, a flaw should happen, its progress might be 

 stopped by drilling a round hole at the extremity of the flaw. These flat glasses 

 may very safely be rubbed with a dry warm cloth. 



As the quantity of electricity excited on glass is nearly in the proportion of the 

 surface exposed to friction ; and as glasses of a great size are very precious, and 

 liable to accidents, I conceived, that instead of a disc of flat glass, one might 

 substitute one of paste-board, thoroughly imbibed with copal or amber varnish. 

 To try how this would answer, about seven years since I ordered 3 paste-board 

 discs to be made, of 4 feet in diameter, the distance of 6 inches from the centre 

 being the fittest to give the whole a proper support in whirling it round. When 

 these discs were thoroughly dried and heated, I poured upon them a varnish 

 made of amber dissolved in linseed oil. After they had taken in as much of the 

 varnish as they could imbibe, I covered them with a thick coat of the same 

 varnish, and dried them by the heat of a German stove. When the varnish 

 was very hard, I found, that even a slight friction with a cat's skin or hare's 

 skin excited a strong electricity on them. 



I then made a frame to place them in, and to whirl them round ; which frame 

 was so contrived, that it could contain about 12 such discs, whirling all round 

 on the same centre. It consisted of 2 square pillars of wood, about 5 feet high, 

 and 3 inches broad ; joined together at top and bottom by a transverse piece of 

 wood. In the middle of the 2 pillars was a hole, about an inch and a half 

 diameter, fitted to receive a wooden axis, which could be placed in, and taken 

 out, at pleasure. On this axis were to be stuck the paste-board discs ; and a flat 

 board, 3 inches broad, covered on both sides with a flannel, and over this with 

 a hare's skin, was to be placed between each paste-board. The 2 square pillars 

 were also to be wrapped up first with flannel, and over that with hare skin. 



The flat boards, to be placerl between the paste-board discs, had each a notch 

 in the centre, to give room for the axis to turn round freely. These flat boards 

 could be brought as near each other as was required by 2 wooden male screws, 

 placed at the u[)per and lower end of the frame, which reached from one square 

 pillar to the other ; which screws were to receive a notch cut out at the upper 



