Farms and Villages. 
57 
momentary pressure should arise in the cylinder, the valves 
open and allow some water to escape, and so prevent disastrous 
strains. Rotatory hydraulic-engines may be used for every 
purpose to which a steam-engine can be applied, and they can 
therefore be used to drive pumps actuated by cranks. But when 
an hydraulic-engine is not required for other work besides pump- 
ing, it is best to adopt a direct-acting motor. Of these there 
are many varieties, but the most simple and satisfactory is one 
arranged on the duplex system. The machine consists of two 
hydraulic pressure-cylinders and two pumps, forming two pairs, 
placed parallel to each other, with a pump in line with a 
cylinder, with their piston-rods connected, so that the pressure 
on the hydraulic-cylinder is communicated direct to the pump. 
The cylinders are fitted with ordinary hydraulic slide-valves, 
without lap, and the pumps are of any variety of the double- 
acting kind. The valve motion consists merely of a pair of 
unequal armed levers, having fixed fulcra in the bed-plate 
which carries the cylinders, and connected so that the long arm 
of one lever is connected to, and worked by the piston-rod of 
the right-hand engine, while the short arm actuates the slide- 
valve of the left-hand cylinder. The other lever is arranged in 
a similar manner to the left-hand piston-rod and the right-hand 
valve. The effect of the arrangement is, that as one piston 
makes a stroke it admits the water pressure to the other cylinder, 
and vice versa. These pumps are extremely simple and durable, 
requiring very little attention ; they may be depended on to do 
70 per cent. duty. 
In dealing with high falls, the pipes are necessarily of consi- 
<Ierable length ; so in the case of turbines, as well as of hydraulic- 
engines, not only must the friction of the rising pipe be taken 
into account in estimating the power, but the friction of the 
supply-pipe of the motor must be ascertained, and its value 
•deducted from the apparent fall. These calculations are not 
difficult, yet it would be prudent in these cases to obtain pro- 
fessional advice. The size and proportions of pressure-engines 
<Iepend so much on the circumstances of particular cases that 
it is impossible to give any general idea of their cost. 
It frequently happens that water motors have to pump dif- 
ferent water from that which drives them. Thus in the case 
of rivers, liable to become turbid or discoloured at times, it is 
often possible, when the strata are sufficiently open and per- 
meable, to construct a natural filter, by digging a trench parallel 
to the river, and a few yards from its margin, filling it with 
•broken stones, or laying in common drain tiles, and then filling 
the trench up again. A small sump or well is constructed at 
one part of the trench, and the suction-pipe of the pump is led 
into it. If the soil be open and porous, and the trench made 
