EXrERlMENTS ON VARIOUS ALLOYS OF GOLD, SOI 



rapid. It was alfo obferved, that the gullies in the upper Description of 

 pieces correfponded to thofe in the lower ones ; fo that it was ^^"in^l* 

 impoffible that the pieces of metal ftiould touch each other in effeds of fric- 

 thofe places where the diminution was raoft rapid, and confe- tlon on com » 

 quently (he gullies mud have been formed by the particles of 

 metal which had been abraded, and which fubfequently had 

 become accumulated. 



It feemed to Mr. C. that the mod probable way to prevent 

 the little furrows or gullies from being thus formed, would be, 

 to conftruct the inftrument in fuch a manner, that the direction 

 in which the pieces rubbed upon each other fliould continually 

 vary. The following contrivance was therefore adopted, by 

 which the pieces were prevented from rubbing together twice 

 in the fame direction. 



In this method, the lower frame, as well as the upper, is 

 fupported on two moveable vertical boards ; but, whereas the 

 boards fupporting the upper frame are placed parallel to Bb, 

 in confequence of which the frame can move only in the di- 

 rection BA, thefe are placed parallel to BA, fo that the frame 

 can move only in the direction Bb. 



EE is the axis by which the upper frame is moved : this 

 turns in fixed fockets at SS, and is turned at each end into the 

 form of an eccentric circle, which acts as a crank ; fo that, by 

 means of the levers EK, which at one end turn on thefe ec- 

 centric circles, and at the other end turn on joints fixed to the 

 upper frame, this frame is made to move $ of an inch, in the 

 direction BA, during one half of the revolution of the axis, 

 and as much in the contrary direction, during the other half 

 revolution. 



ee is an axis of the fame kind, ferving to move the lower 

 frame. HH is a windlafs, which turns thefe two axes by 

 means of the toothed wheels F,/, which work in the toothed 

 wheels G, g f fattened to the axes EE and ee. TTTTT is the 

 table upon which the apparatus Hands. 



The wheel F has 90 teeth, /has 75, and G, g, have eacf> 

 20; fo that the axis EE makes fix revolutions while ee makes 

 five ; and, at a medium, thefe axes make about four revolu- 

 tions to one of the windlafs. A counter is placed fo as to (how 

 the number of revolutions of the windlafs. 



If the two frames had performed their vibrations in the fame 

 time, no advantage would have been gained, for the pieces of 



coin 



