MECHANICS. 61 
which had just been closed by the depression, are agan open, and admit a 
fresh quantity of water, which also is then forced into B; the operation may 
thus be continued for any length of time. It is necessary to mention a 
special contrivance which must be attached to the pumps of this construc- 
tion when the water is to be forced to a great height. The water, as is well 
known, contains a great deal of air mixed with it, which is set free during 
pumping, and collects under the piston. If, now, the column of water 
behind d has a great height, as of 40 or 50 feet. the air in A has to overcome 
a pressure of more than one atmosphere, and thus, mstead of passing out 
through d, becomes compressed by the descending piston, expanding when 
this is elevated, so that when the amount of this air is considerable, it 
becomes impossible to produce a sufficient rarefaction in A to admit of the 
opening of the valves 7, 7, and the ingress of the water. Some plan must be 
resorted to, then, for removing the accumulation of air from the cylinder. 
For this purpose a canal, abc, is bored through the piston, to allow of an 
exit for the air beneath; a screw at a keeps this canal closed. If, now, a 
quantity of air has collected, as indicated by a diminished discharge of 
water, the screw a is to be opened on the descent of the piston, and closed 
when it has reached its lowest point, or when water escapes instead of air 
through the canal abc. 
Suction and forcing-pumps find numerous applications in the arts and 
manufactures, and we shall have frequent occasion to refer to them in the 
technical part of this work. We will here only mention their application 
in hydrology, as, for instance, in the water-works at Marly, where water 
is raised to a height of over 500 feet. Here also belong the fire-engines, 
the largest of which consists of two forcing-pumps, working alternately, and 
driving the water into a larger air-tight vessel, whence it escapes through 
an escape-pipe. A more particular account of various kinds of fire-engines 
will be presented in the tenth division of the work. 
The hydraulic press of Bramah, represented in full on pl. 18, fig. 65, and 
in section of the working part in fig. 66, is another application of the 
forcing-pump. It consists of two principal parts: a forcing-pump which 
exerts a pressure by means of the water raised, and a piston which receives 
the pressure and transmits it, through a plate resting in its upper extremity, 
to any body upon which pressure is to be exerted. The piston, s, is raised 
by the lever, /, and in consequence, the water presses from the reservoir, , 
through the strainer, 7, raises a valve, and thus gets underneath the piston. 
When this piston is depressed, the water closes that valve, opens the valve 
d, and passes through the canal, tbuw, into the cylinder, cc’; here it presses 
against the piston, p, and raises it with the plate p’, so that any body 
between this plate and the fixed plate, e, experiences a great pressure. The 
force with which the smaller piston, s,is depressed, will be to the force 
with which the larger, p, is elevated, as the area of a section of the piston 
s, to the area of a section of the piston p. The amount of force transmitted 
to the piston p, is regulated and measured by a safety valve, g (figs. 67-69). 
Thus knowing the weight, p, the length of the lever arms, fz and fy, and the 
area of the lower surface of the valve, g, the pressure experienced by the valve 
235 
