ain to the capacity
of the smaller drains that fall into it; and this would be the true
rule, were the small drains expected to run full.

If our smallest drain, however, be of two-inch, or even one and a half
inch bore, it can hardly be expected to fill at any time, unless of
great length, or in some peculiarly wet place. Considering, then, what
quantity of water will be likely to be conducted into the main,
proportion the main not to the capacity of all the smaller drains
leading into it, but to the probable maximum flow--not to what they
_might_ bring into it, but to what they _will_ bring.

If the mains be of three-inch pipes, other things being equal, their
capacity is nine times that of a one-inch pipe, and two and a quarter
times the capacity of a two-inch pipe.

A three-inch main may, then, with equal fall and directness, be safely
relied on to carry nine streams of water equal each to one inch
diameter, or two and a quarter streams, equal to a two-inch stream. The
three-inch main will, in fact, from the less amount of friction, carry
much more than this proportion.

The allowance to be made for a less fall in the mains, has already been
adverted to, and must not be overlooked. It is believed that the
capacity of a three or four-inch pipe to convey water, is in general
likely to be much under-estimated.

It is a common error, to imagine that some large stone water-course must
be necessary to carry off so large a flow as will be collected by a
system over a ten or twenty-acre field. Any one, however, who has
watched the full flow of even a three-inch pipe, and observed the water
after it has fallen into a nearly level ditch, will be aware, that what
seems in the ditch a large stream, impeded as it is by a rough, uneven
bottom, may pass through a three inch opening of smooth, well-jointed
pipes. When we consider that a four-inch pipe is four times as capacious
as a two-inch pipe, and sixteen times as large as a one-inch pipe, we
may see that we may accommodate any quantity of water that may be likely
anywhere to be collected by drainage, without recourse to other
materials than tiles.

When one three or four-inch pipe is not sufficient to convey the water,
mains may conveniently be formed of two or more tiles of any form. A
main drain is sometimes formed by combining two horse-shoe tiles, with a
tile sole or slate between them, to prevent slipping, as in fig. 47.

[Illustration: Fig. 47. Fig. 48.

Main D