18 GENERAL CHARACTERS OF PRECIOUS STOiNES 



at c, corresponds to the unit weight, but when placed in the notches of the beam numbered 

 1, 2, 3, &c., it has the vahie ^V-'^' tV-^^' Tiy^i ^c., that is, ^, 3%-, ^,&c., of the unit. A 

 second weight n has one-tenth the value of A^, or n = -^N. When placed on the hook 

 at c its value is ^j^N', and \\hen placed in the notches 1,2, 3, &c., it has the values ^n, 

 Tu'h iTjn, &c., or -y^oN, xtxt-^j tw-^j &c. Lastly, there is a third weight n^ = yV* = 

 TTT-^5 which is its value when suspended from c, but when placed in the notches 1, 2, 3, 

 &c., it has the values ^n^, y^??^, f\yv^, &c., or ^^n, j^7t, y^«, &c., or TTnnr-^> 

 lOoo -^; TTnnr-^' ^^- JL'he weights n and Wj are used only as riders, and of the normal 

 weights, N, one will be required for use as a rider and the others for suspending from the 

 hook under c. For convenience, larger weights, multiples of the normal weight (2iV, 

 3N, &c.), are also supplied for suspending at c. 



The operation of weighing is performed in the following manner : When the beam is 

 horizontal and the pointer indicates zero on the scale at e, let there be a certain number of 

 the normal weights N on the hook c, one of the normal weights in one of the notches on 

 the beam, and weights n and n^ in certain other notches. The number of N weights on the 

 hook c will give the whole number of the required reading, and the riders A', n and % on 

 the beam give the tirst, second, and third decimal places of the same unit. For example, 

 there may be at c three of the weights N, and the riders N, n and n^ at the divisions 7, 2, 

 and 9 respectively ; the reading will then be 3-729 of the unit N, or 3-729A^. If there is 

 110 rider N and riders n, n^ at divisions 3 and 5 respectively, then the reading is 3"035. 

 The arrangement of the weights shown in Fig. 5, corresponds to the reading 3'725A^, and 

 in Fig. 7 to 2-707JV. 



Throughout the whole series of weighings for the determination of a specific gravity 

 the clip for holding the stone and part of the suspending wire must remain immersed in 

 water. The support _/" is adjusted in the tube h so that, during the swinging of the beam, 

 the clip shall nei ther touch the bottom of the vessel nor rise out of the surface of the water, 

 but remain completely immersed in approximately the central part of the vessel. The 

 determination of specific gravity according to this method involves three separate weighings, 

 as described below. One of these may, however, be reduced to a constant and used through- 

 out a series of determinations. 



(1) Sufficient weights are placed on the beam to bring the pointer to zero. 

 The following example illustrates the method of manipulating the weights : The 

 normal weights N, %N, SN, 4iV, successively hung on the hook c were each found to be 

 insufficient to move the beam, but 5N brought it past the horizontal position. One N was 

 therefore removed and -iiVleft on the hook. The rider iVwas then placed on the beam at 

 the divisions 9, 8, 7, &c., successively ; at the third division the beam was still tilted down 

 to the right, but when the rider was at the second division the beam was tilted to the left ; 

 the rider was therefore left at the second division. In the same way the rider n finds a place 

 at 5, and n.^ at the same division, in which case it may hang from the rider n, as shown in 

 Fig. 7, where n.^ hangs from N. The counterpoise of the balance therefore corresponds to 

 the weight 4'255A^. A fourth decimal place in the reading may be obtained by placing 

 the rider n-^^ between the two divisions 5 and 6. If, for instance, it is placed midway between 

 divisions 5 and 6, the reading would be 4'2555A'"; if nearer to 5 than to 6, then it would 

 read 4-2553A^. 



(2) The stone of which the specific gravity is to be determined is now placed in the 

 scale-pan and the weights readjusted until the beam is again horizontal. In the case 

 quoted, the weight required was 3'812A^. Hence the real weight of the stone was 

 (4-255- 3-81 2)iV = 0-443iV. 



