our drains must do a greater work than theirs, and in less time.

There are several other general considerations that should be noticed,
before we attempt to define the particular size for any location.
Several small drains are usually discharged into one main drain. This
main should have sufficient capacity to conduct all the water that may
be expected to enter it, and no more. If the small drains overflow it,
the main will be liable to be burst, or the land about it filled with
water, gushing from it at the joints; especially, if the small drains
come down a hill side, so as to give a great pressure, or head of water.
On the other hand, if the main be larger than is necessary, there is the
useless expense of larger tiles than were required. The capacity of
pipes to convey water, depends, other things being equal, upon their
size; but here the word size has a meaning which should be kept clearly
in mind.

The capacity of round water-pipes is in proportion to the squares of
their diameters.

A one-inch pipe carries one inch (circular, not square) of water, but a
two-inch pipe carries not two inches only, but twice two, or four inches
of water; a three-inch pipe carries three times three, or nine inches;
and a four-inch pipe, sixteen inches. Thus we see, that under the same
conditions as to fall, directness, smoothness, and the like, a four-inch
pipe carries just four times as much water as a two-inch pipe. In fact,
it will carry more than this proportion, because _friction_, which is an
important element in all such calculations, is greater in proportion to
the smaller size of the pipe.

VELOCITY is another essential element to be noticed in determining the
amount of water which may be discharged through a pipe of given
diameter. Velocity, again, depends on several conditions. Water runs
faster down a steep hill than down a gentle declivity. This is due to
the weight of the water, or, in other words, to gravitation, and
operates whether the water be at large on the ground, or confined in a
pipe, and it operates alike whether the water in a pipe fill its bore or
not.

But, again, the velocity of water in a pipe depends on the pressure, or
head of water, behind it, and there is, perhaps, no definite limit to
the quantity of water that may be forced through a given orifice. More
water, for instance, is often forced through the pipe of a fire-engine
in full play, in ten minutes, than would run through a pipe of the sam