[ 337 ] 
means the eleCtric matter found an eafy accefs to the 
place of excitation ; and the effect of the machine 
was thereby incredibly increafed. A piece of lea- 
ther, covered with amalgam, and fixed to the 
pofterior part of the rubber, in fuch a manner as 
to allow about an inch of it to pafs under the cy- 
linder, anfwered every purpcfe of the tinfoil ; and, 
as it was not liable to be corroded by the mercury, 
like tinfoil, it was on that account much preferable. 
From the above experiments it was apparent that 
the excitation was altogether performed by the 
pofterior portion of the cufhion ; and that the an- 
terior part, when made of conducing fubftances, 
re-abforbs the greater quantity of the excited matter. 
In the ftru&ure therefore of electrical machines, 
we fliould always have a free electric communication 
behind, to facilitate the excitation; and the moil 
perfectly nonconducting fubftances before, to pre- 
vent the re-abforption. To anfwer thefe intentions, 
it will perhaps be advifeable to make the cufhion 
of filk, fluffed with hair, and to lay fome metallic 
conductor round the pofterior part, that a free ac- 
cefs might be allowed to the eleCtric matter coming; 
to the place of excitation from the inferior part at 
the machine. Cufhions, made in this manner, 
and then covered with filk ten or twelve times 
doubied, are much more powerfully excitant than, 
any others that I have yet tried. Various other me- 
thods, however, may be purfued in the conftruction 
of the rubber;, but it fliould be an invariable rule, 
to place nonconducting bodies before, and con- 
ducting fubftances behind, the cylinder. From the 
preceding principles, it follows, that the lupport 
to 
3 
