MECHANICS. 67 
when the piston rises. The air compressed in the box E, which serves 
as a reservoir, pours out through a tube attached at m to the fireplace. 
To maintain a uniform stream of air, which is necessary in most smelting 
processes, regulators of various forms are employed. One of these, repre. 
sented in fig. 4, depends for its action upon the pressure of water introduced, 
whence it is called a water regulator. It is very similar in its nature to the 
gasometer previously described: Eis a box consisting of iron plates screwed 
together, containing from 30 to 40 times the volume of the cylinder of the 
blower, and into which the air pours from the cylinder through the tube D, 
escaping again through C. The entire box, E, is suspended equably in a 
cavity of masonwork or iron plates, so as not to touch its bottom. This 
cavity is partly filled with water, which completes the box E. In a state 
of equilibrium, the water will stand at the same Jevel in both vessels ; when, 
however, air is introduced into E from the blower through D, exit through 
C being for the time prevented, the surface of the water in Ii must become 
depressed to rr, while it rises to vv in A. Upon the difference of these two 
surfaces depends the amount of pressure experienced by the air in E, and 
consequently the force of escape through C; which escape is rendered 
uniform by the regulator. If the pressure is to be increased, all that is 
necessary is to increase the height of the water in A by fresh additions. 
It is often necessary to observe and measure the pressure existing in the 
interior of the cylinder, as, for instance, the case might readily occur of an 
escape-valve refusing to do its duty, which might result injuriously, either in 
a bursting of the cylinder, or some other accident. Such results can 
only be avoided by being able to examine at any time the interior pressure. 
For this purpose, the wind measurer, a kind of manometer, has been 
invented. This is represented in pl. 19, fig. 5. It consists of a tin box, 
air-tight and partially filled with water, through whose bottom passes a tube, 
a, which can be attached to the blower by a male screw, and through 
which, therefore, a communication is established between the blower and 
upper part of the box. With the lower part of the latter communicates a 
glass tube, b, provided with a scale, in which, at the beginning, that is, 
before the blowing commences, the water poured in through an opening in 
the cover of the box must stand at the zero of the scale. If, now, by the 
action of the blower, the water in the upper part of the box becomes 
compressed, that in the tube ascends, and by its height indicates the pres- 
sure of air in the blower. At da tube is attached for letting out the water 
in the manometer. 
We will here only add a few words respecting the Jaws which come in 
application in the escape of air. As a general rule, the same laws apply to 
gaseous as to liquid bodies, namely, that the velocities of efflux are as the 
square roots of the heights of pressure, although the latter cannot, as in the 
case of liquid, be determined directly by experiment. In the case of 
liquids we had to do with a pressure column of the same nature and density 
as the escaping liquid; here, however, the pressure is produced by a column 
of air having neither a uniform density nor a fixed limit. In general, how- 
ever, the pressure exerted upon a vessel in escaping is measured by a mano- 
ICONOGRAPHIC ENCYCLOPZDIA.—VOL. I. 16 24] 
