MECHANICS. 37 
second, and weighing half an ounce, to strike a ball of 40 lbs. weight 
suspended to a aia then the common velocity after impact would be to 
1400 
Es : 
1400 as 3 :40+=5 =i hus me 1.09 feet in a second. 
Upon ie principle depends the measurement of great velocities by means 
of the ballistic pendulum. This pendulum, represented laterally (pl. 17, 
fig. 87), and in front ( fig. 38), consists of an iron-bound wooden block, B, 
of considerable weight, which, by means of the iron frame, r, m, s, is attached 
to the’ axis, C, in such a manner that it can swing about this axis, which is 
supported at D. Above is attached a graduated arc, no, on which an index 
shows the amplitude of oscillation; beneath is an arched piece containing a 
groove filled with soft wax, on which the index, f, in the motion of the pen- 
dulum, makes a scratch, exhibiting graphically the length of oscillations when- 
ever a ball, A, strikes the pendulum in the direction of the centre of gravity. 
The pendulum is 10—12 feet in length. To determine the velocity of a cannon 
ball it is fired against the pendulum, and its motion is thus communicated to 
the latter. Knowing the are described by the pendulum, as well as the 
mass of both pendulum and ball, it is a simple problem to ascertain the 
velocity of the ball. 
C. Sratics or Fiurps.—Hyprostatics. | 
a. Pressure of Liquids. 
As the statics of solid bodies had reference to the laws of their equilibrium, 
hydrostatics embraces the theory of equilibrium in liquids, and of the pres- — 
sure which they exert upon the walls of the containing vessel. 
In liquid bodies, two forces are to be considered, namely, weight and 
molecular attraction; and these two forces may be readily imagined to be 
separated from each other, that is, a liquid may be supposed to exist without 
weight. Such a liquid left to itself would not fall: it thus needs no support 
on any side, and might even sustain a pressure and transmit it according to 
a certain principle. Hence the following axiom: a liquid transmits pres- 
sure acting upon any part of its surface, uniformly in every direction. 
Suppose a vessel to contain such a liquid, with a suitable piston, also without 
weight, placed upon its surface. The liquid would not flow out, even if the 
side of the vessel were pierced by an aperture. If, however, a weight be 
placed upon the piston, it would sink if not supported by the liquid, whose 
upper layer would likewise sink unless supported by the one beneath it, and 
so on to the bottom of the vessel. All these layers of liquid, therefore, 
receiving successively the same pressure, the result is the same as if the 
piston with its superincumbent weight pressed directly upon the bottom of 
the vessel. Hence it follows, that the pressure upon horizontal surfaces is 
transmitted from above to below without any loss, that is, is equal at every 
point, and proportional to the surface involved. 
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