302 EXPERIMENTS ON VARIOUS ALLOYS OF GOLD.' 



Defcription of coin would ftill have moved upon each other always in the 

 machinery for f ame jj ne but as (he ; r vl brations are performed in different 

 determining the ' r 



effeds of fric- times, Mr. C. (hews, that the effect muff be quite different, 

 tioa on coin. T n ; s contrivance, therefore, effectually prevented the pieces 



from moving upon each other always in the fame line ; and it 

 feems alfo to have much diminished the difpofition which they 

 had to wear in gullies, but not intirely ; for, from the following 

 experiments it appears, that ftill fome few particles would be- 

 come occafionally collected, and then a<5ted as a grinding 

 powder, which accelerated the wear of the pieces. This was 

 obferved particularly to liappen to the pieces of gold alloyed 

 with an equal proportion of copper, and to the pieces of 

 copper, which were alfo more frequently worn in furrows or 

 gullies, than the other pieces of more ductile metal. 



The motion of the pieces of coin upon each other, is greater 

 than it would have been if only one frame had been made to 

 move, nearly in the proportion of 3 to 2 ; fo that the whole 

 motion of the pieces, in each femi-revolution of the axes EE 

 or ee, is about -| of an inch, and therefore it is about three 

 inches in each revolution of the windlafs. 



The inftrument employed in the fecond feries of experi- 

 ments, is fo fimple as not to require any drawing. It con- 

 fided only of a cubical box of oak, which meafured eight 

 inches each way, within tide. This box was moved by the 

 axis EE of the former inftrument, which was palled through 

 the middle of two oppofite fides, and was fixed in that pofi- 

 tion. 



Fig. 4, reprefents a plan of the inftrument ufed in the third 

 feries of experiments, aaa is a horizontal table, turning upon 

 a vertical axis ; and BBBbbb is a fixed frame funounding it. 



The pieces of coin are fattened to this fixed frame, by the 

 fame connecting pieces which were formerly employed, and 

 are prefled down alfo by fimilar weights. The diameter of 

 that part of the wheel againft which the centres of the pieces 

 of coin are prefled, is 29 inches; fo that, while this wheel 

 makes one revolution, the pieces are rubbed againft it through 

 the whole circumference of this circle, that is, through 91 -— 

 inches 



- A fhallow groove ggg is cut in this wheel, in that part againft 

 which the pieces are prefled, in order to confine the powders 

 employed in the experiments; and the number of revolutions of 

 the wheel are marked by a counter. 



By 



