300 EXPERIMENTS ON VARIOUS AtLOYS OP GOLD. 



Defcription of 1ft. That the piece? of coin fhould all move equally. > 



machinery for 2d j That th ftlou | d aJ1 be pre fl- ed a g a j n ft. the lower 



determining the . , . ,. / . . , , 



effe&soffric- pieces by the lame weight. And, 



two on coin. 3dly. That they mould bear flat againft them. 



As to the firft requifite, it is evident that the pieces muft all 

 move alike, excepting fo far as proceeded from the fpringing 

 of the parts of the machine, or from the make in its joints, 

 both of which were very fmall. 



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 prefled againft the lower one, depends only on its 

 own weight, on that of the focket w, on that of the connecting 

 piece Nn, and on the weight Z by which it is loaded ; fo that 

 the fecond requifite is thus eafily obtained. 



Thirdly, The connecting piece N/i bears againft the focket w 

 only by the pin p, which enters into a hole in the centre of the 

 focket, fo that the two pieces muft neceflarily bear flat againft 

 each other; but, as this pin alone would not have prevented 

 the focket from turning round on its centre, two other pins 

 n II were fixed into the connecting piece, and entered into 

 flits made in the focket near its circumference, allowing no 

 more fliake than was neceflary to prevent it from flicking; and 

 thus the motion round the centre was effectually prevented. 



It may be obferved, that the pieces might have oeen made 

 to bear flat againft each other by fixing the fockets w in gim- 

 bals ; but, as the method above defcribed was effectual, and 

 much eafier made, it was preferred. 



It may be alfo remarked, that the breadth of the bars Cc, as 

 reprelented in Fig. 1, is not fufficient to prevent them from 

 fpringing considerably ; for this reafon, a method of ftrength- 

 ening them was employed, which anfwered the purpofe per- 

 fectly well, but is omitted in the drawing, as it could not be 

 eafily reprefented. 



It was at firft intended, that the lower frame fhould have re- 

 mained fixed, and that only the upper one fhould have moved; 

 but, in a previous trial, in which two pieces of metal were 

 rubbed backwards and forwards upon each other in the fame 

 line, with a view to difcover what weight would be neceflary 

 to make the pieces wear tolerably faft, it was found that for a 

 time they diminiflied flowly, but that little furrows or gullies 

 were foon worn in them, and that then the diminution was 



rapid 



