specific Gravity, and comparative Wear of Gold. 143 
each other ; but, as this pin alone would not have prevented the 
socket from turning round on its centre, two other pins ri n 
were fixed into the connecting piece, and entered into slits made 
in the socket near its circumference, allowing no more shake 
than was necessary to prevent it from sticking ; and thus the 
motion round the centre was effectually prevented. 
It may be observed, that the pieces might have been made to 
bear flat against each other by fixing the sockets zv in gimbals ; 
but, as the method above described was effectual, and much 
easier made, it was preferred. 
It may be also remarked, that the breadth of the bars C c, as 
represented in Fig. 1, is not sufficient to prevent them from 
springing considerably; for this reason, a method of strengthening 
them was employed, which answered the purpose perfectly well, 
but is omitted in the drawing, as it could not be easily represented. 
It was at first intended, that the lower frame should have re- 
mained fixed, and that only the upper one should have moved ; 
but, in a previous trial, in which two pieces of metal were 
rubbed backwards and forwards upon each other in the same 
line, with a view to discover what weight would be necessary 
to make the pieces wear tolerably fast, I found that for a time 
they diminished slowly, but that little furrows or gullies were 
soon worn in them, and that then the diminution was rapad. I 
also observed, that the gullies in the upper pieces corresponded 
to those in the lower ones ; so that it was impossible that the 
pieces of metal should touch each other in those places where 
the diminution was most rapid, and consequently the gullies 
must have been formed by the particles of metal which had 
been abraded, and which subsequently had become accumulated. 
