t would be $200 cheaper,
therefore, to borrow $500, at 5 per cent, to pay for a gravity supply
rather than to pay $30 for a pump which costs ten cents a day to run.
This same reasoning may be applied to the cost of different kinds of
pumps. One pump may cost $200 more than another, but the saving in fuel
and repairs may be sufficient to more than justify this additional cost.

Second, a gravity supply is to be preferred because of its greater
reliability. It is hardly possible to imagine any excuse for a gravity
supply failing to deliver its predetermined quantity of water regularly
day after day. A pumping plant, on the other hand, both breaks down and
wears out. Valves are continually requiring to be repacked, nuts drop
off and have to be replaced, pieces of the machinery break and require
repairs, so that with the best machinery it is almost inevitable that
for many days in the year the water-supply is interrupted by some
failure of the machinery. In planning water works for cities, an
engineer weighs and estimates the value of a continuous service, and
even if the gravity supply costs somewhat more than the pumping system,
it is in many cases adopted because the greater cost is supposed to be
compensated for by the greater reliability of the supply.

_Windmills._

Perhaps the cheapest source of power for pumping water is a windmill,
and in many cases it proves entirely serviceable. It has two drawbacks
which are self-evident. Unless the wind blows, the mill will not work,
and, unfortunately, at those times of the year when a large supply of
water is most to be desired, that is, during the hot summer months, the
wind is particularly light. It is necessary, therefore, when using wind
as a source of power, to provide large storage which will tide over the
intervals between the times of pumping. Again, the wind may blow
frequently enough, but may be so light as not to turn the large vanes
necessary to pump rapidly and easily the large amount of water needed.
Nothing less than a twelve-foot mill ought to be erected, and, to be
efficient, the wind must blow at the rate of twelve to sixteen miles an
hour.

[Illustration: FIG. 43.--Windmill and water tank.]

A windmill of the best design is made entirely of steel with small angle
irons for posts for the tower, and with the mill itself made of
galvanized iron. It requires a good foundation and must be well anchored
to the masonry piers by strong bolts set well down into