142 Mr. Hatchett’s Experiments on the various Alloys , 
is bedded and fixed firmly in a brass socket x, fastened to the 
lower frame ; u is the piece of coin to be rubbed against it, 
which, in like manner, is fixed in another brass socket w ; N n 
is the connecting piece, by which this socket is connected to the 
bar C of the upper frame. This piece turns on pivots, in two 
studs n, fixed to the bar C, so that it can turn freely on those 
pivots in a vertical direction, but cannot be perceptibly shaken 
horizontally. 
Z is the weight by which this connecting piece is pressed 
down; it is round, and is placed with its centre exactly over 
that of the socket w. 
It must be observed that, in the construction of this machine, 
three things principally demanded attention. 
ist. That the pieces of coin should all move equally, 
adly. That they should all be pressed against the lower 
pieces by the same weight. And, 
gdly. That they should -bear flat against them. 
As to the first requisite, it is evident that the pieces must all 
move alike, excepting so far as proceeded from the springing of 
the parts of the machine, or from the shake in its joints, both of 
which were very small. 
Secondly, as the connecting pieces move freely in a vertical 
direction, it is clear that the force with which the upper piece of 
coin is pressed against the lower one, depends only on its own 
weight, on that of the socket w, on that of the connecting 
piece Nzz, and on the weight Z by which it is loaded ; so that 
the second requisite is thus easily obtained. 
Thirdly, the connecting piece N n, bears against the socket w 
only by the pin p, which enters into a hole in the centre of the 
socket, so that the two pieces must necessarily bear flat against 
