HYDRAULICS. 
In this contrivance the piston, A, see fig. 
9, lias a valve which, as the rod draws up, 
is closed by the pressure of the air above 
it; but in descending it opens, and allows 
the water, which laid flowed into the lower 
part, whence the air was withdrawn, to 
rush through ; as the piston is raised again, 
the weight of the water forcibly oppresses 
Iho valve, until it find a lateral passage at 
B, whence it issues, and in this manner any 
quantity may be raised. If the water has a 
direct issue, as in the common spouts of 
pumps, no further apparatus is wanted ; but 
if it is to be retained, or pass through any 
other pipes more elevated than the debou- 
chure, B, there must be a small angular 
projection, as shown by the dotted lines, to 
admit the valve C, also pointing upwards. 
In dry weather, or when the. pump is not 
much used, the leather binding of the pis- 
ton, as also the valves, will become dry ; 
therefore it is necessary, on such occasions, 
to throw in a pail-full or two of water to 
moisten them; else the air will pass down- 
wards as the piston rises, and prevent that 
exhaustion on which the accent of the water 
depends. It is generally necessary to have 
a valve at the bottom of the pipe to keep 
in the water drawn into it, in order that 
the labour may be decreased ; and that, if 
the pumping be intermitted, there may be 
less trouble in bringing up the water within 
reach of the piston. 
Where the water lays near the surface, a 
lifting-pump may lie used. This is nearly the 
same with the former; but requires the pis- 
ton should lie forced down beneath the level 
of the water in the well. In this it is not 
so indispensably necessary that the leather 
on the piston should fit so close : though it 
is the better for so doing. In the lifting- 
pump the whole depends on actually raising 
the water from the well as though it were 
done by means of a bucket; this occasions 
many to apply that designation to the pis- 
ton. The same precautions are necessary 
if the water is to be passed into any pipe, 
as have been stated regarding the debou- 
chure of the sucking-pump. 
The forcing-pump has a solid piston, as 
seen at A in fig. 10, which, after the water 
has passed the valve at R, is pressed down, 
and causes the fluid to pass into the con- 
ducting pipe, C, where there is also a valve, 
d, to prevent its return. The valve at B 
closes as the piston descends, while that at 
d rises to allow its escape from the main 
pipe. When the piston rises, the water fol- 
lows, as in the two former instances, through 
VOL. III. 
the lower valve B, while the smaller valve 
at d is also closed by the super-incumbent 
water in the conduit, <?, and by the attrac- 
tion of the piston, the water rushing after it 
to prevent a vacuum. In this kind of pump 
the piston must fit extremely close ; both on 
account of the intended attraction of the 
fluid from be'iow, and to prevent its escape 
upwards when the piston is pressed down- 
wards. 
The whole of those inventions which raise 
water by alternate risings and fallings of 
only one piston are subject to the inconve- 
nience of having the water issue in jerks, 
which, in some instances, would prove highly 
inconvenient. To remedy tiiis, a cistern 
should be placed near the debouchure, or 
spout, whence a small stream would flow 
with much less variation than from the spout 
itself. But the best mode of regulating the 
issue of water is by aid of an air-vessel, as 
in a fire-engine. See Pneumatics. 
To detail all the varieties of pumps that 
are in use would be both beyond the limits 
of this work, and of no real utility to the 
reader : we shall therefore enter upon the 
description of the valves in general estima- 
tion, and then proceed to give a brief ac- 
count of hydraulic machinery. 
The most common kind of valve consists 
of a piece of stiff leather, such as is applied 
for soies in shoes, and is generally known by 
the name of pump-leather, On its upper 
side a piece of milled lead is rivetted firmly, 
and the part where it is to be fixed on the 
frame, or shell, of the piston, is grooved for 
the purpose of giving it pliancy, that it may 
work up and down as if on a hinge. Fig. 1 l' 
shows the plan ; and fig. 12, the profile of 
this valve, which is cheap, simple, and easily 
repaired, though it has the defect of being 
liable to choke, and of not rising high 
enough to allow a sufficient passage for the 
water. 
Fig. 13, shows a button-valve, which is 
merely a piece of turned metal, A, having a 
shank, B, of about eight inches or a foot in 
length, according to the depth of the block, 
x z. The shank passes through the bar, C, 
at the bottom of the block, and is prevented 
from coming up too high by the stud, or 
nut, o, at its bottom. When the water 
rises, it forces up the button, A, and passes 
through the hollow in the block, of which 
the superior part is expanded so as to fit the 
button, which being the frustrum of a cone, 
necessarily fits close into the expanded part 
as the water presses it, after having passed 
upwards in consequence of the descent *f 
LI 
