58 
On Water Supplies suited to 
large enough, sufficient water will filter through to give a 
uniformly good supply. Sometimes the porous strata are too 
deep for a trench, and in such case a well may be sunk and 
made to serve the same purpose. 
Hydraulic-rams are constructed specially so as to pump a 
different water from that which works them. To accomplish 
Fig. 8. — Elevation (sails closed) of the Halladay Windmill. 
this, the inner valve D (Fig. 6) is replaced by a small cylinder, 
fitted with a well-packed piston of very short stroke. The 
piston rests on stops at the bottom of its stroke, and is further 
pressed down by a light spiral spring. The passage leading up 
to the air-vessel is fitted with two valves, one over the other, the 
space between being connected with that over the piston above 
described. The two valves open upwards, and the suction-pipe 
from the pure water source is connected to the space under the 
