106 On the Generation of Statical Electricity. 
The cushions should be made quite firm, and not stuffed with 
hair, as that does not allow them to offer sufficient resistance ; 
they should be as perfect non-conductors as possible; the backs 
of the cushions should be of glass or well-baked wood, in order to 
prevent the cushion from abstracting a portion of the electricity 
generated by the rubber. <A piece of smooth white leather, about 
three times the breadth of the cushion, should be fastened by 
one of its edges to the glass or wooden back of the rubber, and 
passing over the face of the cushion, will thus be pressed against 
the glass, the remaining edge being loose. ‘This flap of leather 
should not be very thin, otherwise there will be a useless adhe- 
sion to the glass, increasing the amount of friction unnecessari- 
ly; it should be kept perfectly dry, as the entire action of the 
machine, be it ever so powerful, will be destroyed should it be- 
come damp on the surface opposed to the glass. ‘T’o this leather 
flap, and opposite to the centre line of the cushion, fasten the 
strip of amalgamated leather by means of alittle warm bees-wax ; 
the face of the leather will then exhibit the appearance as shown 
by the adjoining figure: the projecting end of the rubber proper 
being left, for the purpose of forming a metallic com- 
munication between it and the exterior coating of a 
Leyden jar, or with the earth. 
To obtain the maximum effect, those portions of 
the leather flap on each side of the amalgam strip, 
which are pressed against the glass by the cushion, 
should be touched with a little bisulphuret of tin, 
which, in the larger machines, will be found to increase 
powerfully the action ; it requires to be renewed, how- 
ever, every twenty minutes, whilst the machine is being worked ; 
at each of which renewals, the amalgam should be wiped clean, as 
the sulphuret of mercury, which may be formed, is detrimental. 
in its action. It may be well to observe, in this place, that the 
glass should be oiled and wiped clean before a course of experi- 
ments, to prevent its surface from attracting moisture. 
The rubber being finished, it must be thoroughly dried to ex- 
pel any moisture, before being applied to the machine. The ordi- 
nary degree of firm pressure, employed in the common-sized ma- 
chines turned by means of a crank, has been found to produce 
nearly the maximum degree of excitement ; any increase of pres- 
sure above this limit, although generally followed by an increas- 
