42 PHYSICS. 
weighed upon the scale at D, and restore equilibrium, so that the diflerence 
shall be less than one grain. If the entire balance be raised or depressed, 
by means of the apparatus, C, until equilibrium is perfect, and if the index, 
F, point exactly to the middle of the scale cylinder, then the weights laid in 
D' exactly represent that of the body in question. If, however, the index, F, 
point above or below the middle of the scale, as, for instance, to 36, then 2.5,ths 
of a grain are to be added to or subtracted from the weight already ascer- 
tained, as the case may be, to determine the absolute weight of the body in 
D. To determine the specific gravity, again attach the bulb or cup, P’, 
restore equilibrium, and then place the body to be examined in P’. The 
equilibrium again restored by weights placed in D’, and the indications of 
the index, F’, will give the weight of the water displaced. 
The specific gravity of solids may also be determined by means of 
Nicholson’s areometer (pl. 18, fig. 15), which, by an error of the engraver, 
is represented inverted, and consequently requires an inversion of the plate 
to bring it right again. A small heavy mass, as a glass ball, filled with 
mercury, is suspended to a hollow glass body, V, whose upper part on 
immersion must project above the surface of the liquid. To the upper part 
is attached a fine rod, f, which carries a small pan, c. Lay upon this the 
body to be examined, and cause it, by means of additional weights, if neces- 
sary, to sink to a point, f. Remove the body from the pan, and substitute 
as many weights as will bring the point p of the areometer back again to 
the surface of the water: these additional weights give the absolute weight 
of the body, equal, we will suppose, to n. Remove the weights, n (not, how- 
ever, those previously imposed), and place the body in a little basket between 
V andl. The instrument will not sink to f, this requiring the addition of 
weights in the upper pan. The amount of these latter weights = m, 
n 
will give the weight of the liquid displaced, and the specific gravity = —. 
To determine the specific gravity of fluids, a scale areometer ( pl. 18, fig. 
16), may be employed. This consists of a cylindrical glass tube, in the 
lower part of which a ball, 6. is blown, which is continued into a smalier 
tube, terminating finally in another ball, c. This latter ball is filled with 
shot or mercury sufficiently to cause the instrument to sink vertically in 
distilled water to a certain point, the zero. In any other liquid the instru- 
ment will sink until its weight is equal to that of the liquid displaced ; deeper, 
therefore, as the liquid is lighter: so that the specific gravity of the liquid 
can be ascertained by the depth of depression. For this purpose, the areo- 
meter of Gay Lussac has the point, a, at which it stands in water, indicated 
by 100, and upon the tube above and below this point, a divided scale 
attached, so that the volume of the tube included between any two divisions 
of the scale is ;4,th of the volume sir.king in the water, the numeration being 
carried from below upwards. An aceometer divided in this manner is called 
a volumeter. The specific gravity of a liquid is ascertained by introducing 
the instrument and dividing 100 by the number on the scale to which it 
sinks. A volumeter of this character is the more sensitive as the distance 
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