•EXPERIMENTS ON VARIOUS ALLOYS OP GOLD. 299 



The lower frame confifts of a board, placed immediately D* fcription of 

 under the upper frame, and is expreffed in Fig. 2, by the letters determining die 

 LL. effe&s of fric- 



The upper frame is fupported by two vertical boards, ex- tl0I? on C0IIU J 

 lending the whole length of the tides B6 and Aa, fo that the 

 ends of them are (e'en in Fig. 2, and are denoted by the letters 

 DD, DD. Thefe boards are fattened to the upper frame, and 

 to the table upon which the apparatus ftands, by hinges, fo 

 that the upper frame can move freely in the direction BA, but 

 can nave no motion in the direction perpendicular thereto. 

 Thefe vertical boards are omitted in Fig. 1 ; for, as the inten- 

 tion of this defcription is not to give a detail of all the parts of 

 the inuruments, but only to explain their manner of acting, I 

 have taken the liberty to omit fuch parts as tended to produce 

 an intricacy in the figures, without being neceilary to this 

 object. 



Tire difpofition of the pieces of coin on the frames, is re- 

 prefented in Fig. 1 . Nnn denote one of the connecting pieces, 

 by which the upper pieces of coin are connected to the upper 

 frame, and in which the fmall circle reprefents the pofition of 

 the coin ; the large circle is the part which fupports the weight, 

 and nn the part by which it is connected to the upper frame. 



To avoid confufion, neither thefe connecting pieces nor the 

 pieces of coin are reprefented in Fig. 2 ; but, inftead thereof, 

 a fection of one of thefe pieces is given in Fig. 3, upon a larger 

 fcale. ,. 



In this figure, LL is the lower frame, and C one of the bars 

 of the upper frame ; y is one of the lower pieces of coin, which 

 is bedded and fixed firmly in a brafs focket x, fattened to the 

 lower frame ; u is the piece of coin to be rubbed againft it, 

 which, in like manner, is fixed in another brafs focket ry;.->N» 

 is the connecting piece, by whjch this focket is connected t 

 the* bar C of the upper frame. This piece turns on pivots, in 

 two fiuds n, fixed to the bar C, fo that it can turn freely on 

 thofe pivots in a vertical direction, but cannot be perceptibly 

 fhaken horizontally. 



Z is the weight by which this connecting piece is prefled 

 down ; it is round, and is placed with its centre exactly over 

 that of the focket w. 



It muft be obferved that, in the construction of this machine, 

 three things principally demanded attention. 



l/hThat 



