:2^4 A/r. Nicholson’s Experiments and 
Turn the cylinder till the fiik flap has wiped off fo much 
of the greafe as to render it femi-tranfparent. 
Put fome amalgam on a piece of leather, and fpread it well 
fo that it may be uniformly bright. Apply this againft the 
turning cylinder. The friction will immediately increafe, and 
the leather muft not be removed until it ceafes to become 
greater. 
Remove the leather, and the action of the machine will be 
very ftrong. 
, My rubber, as before obferved, confifts of the filk flap parted 
to a leather, and the cufhion is prefled againft the filk by a 
{lender fpiral fpring in the middle of its back. The cuftiion 
is loofely retained in a groove, and refts againft the fpring only, 
in fuch a manner that by a fort of libration upon it as a ful- 
crum, it adapts itfelf to all the irregularities of the cylinder, 
and never fails to touch in its whole length. There is no ad- 
justment to vary the preflure, becaufe the preflure cannot be 
too fmall when the excitation is properly made. Indeed, the 
adlual withdrawing of the cufhion to the diftance of one-tenth 
of an inch from the filk, as in par. 2. will not materially affedl 
a good excitation. 
The amalgam is that of Dr. Higgins, compofed of zinc 
and mercury. If a little mercury be added to melted zinc, it 
renders it eafily pulverable, and more mercury may be added to 
the powder to make a very foft amalgam. It is apt to cryftal- 
lize by repofe, which feems in fome meafure to be prevented by 
triturating it with a fmall proportion of greafe : and it is 
always of advantage to triturate it before ufing. 
A very ftrong excitation may be produced by applying the 
amalgamed leather to a clean cylinder with a clean filk. But 
4 * it 
