purposes.

THE HOUSEHOLD FILTER.

When there is any doubt as to the wholesomeness of the water supply, a
small filter is often used. The microbe-stopper is usually either
charcoal, sand, asbestos, or baked clay of some kind. In Fig. 185 we
give a section of a Maignen filter. R is the reservoir for the filtered
water; A the filter case proper; D a conical perforated frame; B a
jacket of asbestos cloth secured top and bottom by asbestos cords to D;
C powdered carbon, between which and the asbestos is a layer of special
chemical filtering medium. A perforated cap, E, covers in the carbon and
prevents it being disturbed when water is poured in. The carbon arrests
the coarser forms of matter; the asbestos the finer. The asbestos jacket
is easily removed and cleansed by heating over a fire.

[Illustration: FIG. 185.]

The most useful form of household filter is one which can be attached to
a tap connected with the main. Such a filter is usually made of
porcelain or biscuit china. The Berkefeld filter has an outer case of
iron, and an interior hollow "candle" of porcelain from which a tube
passes through the lid of the filter to a storage tank for the filtered
water. The water from the main enters the outer case, and percolates
through the porcelain walls to the internal cavity and thence flows away
through the delivery pipe.

Whatever be the type of filter used it must be cleansed at proper
intervals. A foul filter is very dangerous to those who drink the water
from it. It has been proved by tests that, so far from purifying the
water, an inefficient and contaminated filter passes out water much more
highly charged with microbes than it was before it entered. We must not
therefore think that, because water has been filtered, it is necessarily
safe. The reverse is only too often the case.

GAS TRAPS.

Dangerous microbes can be breathed as well as drunk into the human
system. Every communication between house and drains should be most
carefully "trapped." The principle of a gas trap between, say, a kitchen
sink and the drain to carry off the water is given in Fig. 186. Enough
water always remains in the bend to rise above the level of the elbow,
effectually keeping back any gas that there may be in the pipe beyond
the bend.

[Illustration: FIG. 186.--A trap for foul air.]

WATER-ENGINES.

Before the invention of the steam-engine human industries were largely
dependent on the motive power of the wind and run