Before the piston has arrived at the bottom of the cylinder, the lifter
will have lifted the weighted lever from the buffer-box, and raised it
to a vertical position; from there it will have fallen on the right-hand
key-arm, and have brought the cock-key to its former position, ready to
begin another upward stroke.

[Illustration: FIG. 184.]

The _index mechanism_ makes allowance for the fact that the bevel-wheel
on the pinion shaft has its direction reversed at the beginning of every
stroke of the piston. This bevel engages with two others mounted loosely
on the little shaft, on which is turned a screw thread to revolve the
index counter wheels. Each of these latter bevels actuates the shaft
through a ratchet; but while one turns the shaft when rotating in a
clockwise direction only, the other engages it when making an
anti-clockwise revolution. The result is that the shaft is always turned
in the same direction.

WATER-SUPPLY SYSTEMS.

The water for a town or a district supply is got either from wells or
from a river. In the former case it may be assumed to be free from
impurities. In the latter, there is need for removing all the
objectionable and dangerous matter which river water always contains in
a greater or less degree. This purification is accomplished by first
leading the water into large _settling tanks_, where the suspended
matter sinks to the bottom. The water is then drawn off into
_filtration beds_, made in the following manner. The bottom is covered
with a thick layer of concrete. On this are laid parallel rows of
bricks, the rows a small distance apart. Then come a layer of bricks or
tiles placed close together; a layer of coarse gravel; a layer of finer
gravel; and a thick layer of sand at the top. The sand arrests any solid
matter in the water as it percolates to the gravel and drains below.
Even the microbes,[36] of microscopic size, are arrested as soon as the
film of mud has formed on the top of the sand. Until this film is formed
the filter is not in its most efficient condition. Every now and then
the bed is drained, the surface mud and sand carefully drained off, and
fresh sand put in their place. A good filter bed should not pass more
than from two to three gallons per hour for every square foot of
surface, and it must therefore have a large area.

It is sometimes necessary to send the water through a succession of
beds, arranged in terraces, before it is sufficiently pure for drinking