specific Gravity , and comparative Wear of Gold. 1 47 
are pressed, is 2 9 inches; so that, while this wheel makes one 
revolution, the pieces are rubbed against it through the whole 
circumference of this circle, that is, through 91^ inches. 
A shallow groove ggg is cut in this wheel, in that part against 
which the pieces are pressed, in order to confine the powders 
employed in the experiments ; and the number of revolutions of 
the wheel are marked by a counter. 
By the help of the instruments above described, it was pro- 
posed to determine, as accurately as possible, 
1st. The comparative wear of soft and of hard gold. 
2dly. Whether coins with flat or with raised surfaces suffer 
the greatest loss by friction, when subjected to it under similar 
circumstances.* 
It is scarcely necessary to observe, that rigorously exact 
results could not be expected in all the minutiae of experiments 
like the present; for, many circumstances, apparently but trivial, 
produced almost unsurmountable obstacles ; but, nevertheless, 
these did not impede the essential objects from being investi- 
gated, and determined, in a manner sufficiently satisfactory. 
Before the experiments are described, it will be proper to add, 
that, to obviate the irregular effects which would be produced 
by the inequality of the impressions usually employed for 
coins, Mr. Cavendish suggested a die, which was executed by 
Mr. Pingo, and which consisted of round prominencies re- 
gularly disposed over the surface, so that the effects which this 
* Although coins with protuberances on their surfaces, have been generally sup- 
posed to suffer more by friction than those which are flat, yet, as this opinion has 
been questioned, and as several objections have been made to it by intelligent persons, 
it was thought expedient that the decision of the question should form part of the 
present investigation 
U 2 
