MECHANICS. 4) 
shorter, and provided with a little hook beneath, from which the body 
whose specific gravity is to be ascertained, may be suspended. The abso- 
lute weight of the body thus suspended, is first to be ascertained by weighing 
it in the air, the weight being placed in the opposite scale. Place a vessel, 
D, filled with distilled water under C, and allow the body to be completely 
immersed in it, taking care to remove all air bubbles from its surface, its 
weight will of course be diminished, and to restore equilibrium, weights 
must be placed in C, or removed from D. The amount of these weights 
indicates the loss experienced by the body in its immersion, and consequently 
the weight of a mass of water equal in volume to that of the body itself. 
The specific gravity of the body is the quotient arising from dividing the 
absolute weight by the weight of an equal volume of water, or the loss of 
weight experienced when immersed in the water. 
A very well adapted and useful hydrostatic balance is represented in pl. 
18, fig. 13, giving a front view, and fig. 14, one from the side. To the 
main pillar, A, an arm is attached above, containing two pulleys, over which 
strings pass supporting a small beam to which the balance is suspended. 
The strings are united together into one behind the pulleys, and by means 
of the screw arrangement, C, may be drawn up or let down, the whole play 
amounting to 1—2 inches. The shears of the balance beam are pierced 
above, for the purpose of showing the point of the tongue, and thus deter- 
mining whether equilibrium be attained or not. To the balance beam, B, 
are suspended the two scale-pans with small hooks beneath. DD’ is a thin 
plate attached to a special support beneath the scale-pans, admitting of being 
raised or depressed at pleasure. This plate, DD’, is pierced to allow pas- 
sage to the brass wires attached to the hooks beneath the scale-pans. To 
the wire at D is attached a thin brass cylinder, pierced below, to allow any- 
thing to be suspended from it. This cylinder, about five inches long, is 
covered with paper, upon which an equally divided scale is drawn. In one 
corner of the plate,.DD’, a wire passes with considerable friction through 
an aperture ; to its lower end the index, I’, is attached, which, by the fric- 
tion of the wire in the hole, can be placed at any desired position with 
reference to the scale. At the lower end of the scale cylinder is attached | 
a weight, G, and to this, by means of a fine wire, the brass ball, P, of about 
4-inch in diameter. To D’ is suspended, by a horse-hair, the large hollow 
glass bulb, P’. : 
Suppose the weight, G, to be removed, and the wire with P attached 
directly to the cylinder; suppose P’ also to be replaced with a weight, z, 
heavy enough to produce an equilibrium with the other scale and its ap- 
pendages, when the middle of the wire, with P attached, is intersected by 
the surface of the water. The wire to which P is attached must weigh 
exactly four grains to the inch. As brass is about eight times as heavy as 
. water, the wire will lose half a grain for every inch immersed in the water. 
If. then, everything be in equilibrium when the centre of this wire lies on the 
surface of the water, and if the index, I", lie against the middle of the scale 
cylinder, divided into 100 equal parts, the weight of a body can be ascer- 
tained accurately to within ;3,;th of a grain. Thus, lay the body to be 
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