the pump is to be operated by an engine, the strength of
the latter would have to be 8.7 H.P. An 8.7-H.P. pumping engine
working at full speed every second of the day and night would be able
to supply the town with the necessary amount of water. When, however,
we consider the actual height to which the water is raised above the
pumping station, and the extra pumping which must be done in order to
balance the frictional loss, it is easy to understand that in actual
practice a much more powerful engine would be needed. The larger the
piston and the faster it works, the greater is the quantity of water
raised at each stroke, and the stronger must be the engine which
operates the pump.

In many large cities there is no one single pumping station from which
supplies run to all parts of the city, but several pumping stations
are scattered throughout the city, and each of them supplies a
restricted territory.

199. The Bursting of Dams and Reservoirs. The construction of a safe
reservoir is one of the most important problems of engineers. In
October, 1911, a town in Pennsylvania was virtually wiped out of
existence because of the bursting of a dam whose structure was of
insufficient strength to resist the strain of the vast quantity of
water held by it. A similar breakage was the cause of the fatal
Johnstown flood in 1889, which destroyed no less than seven towns, and
in which approximately 2000 persons are said to have lost their lives.

Water presses not only on the bottom of a vessel, but upon the sides
as well; a bucket leaks whether the hole is in its side or its bottom,
showing that water presses not only downward but outward. Usually a
leak in a dam or reservoir occurs near the bottom. Weak spots at the
top are rare and easily repaired, but a leak near the bottom is
usually fatal, and in the case of a large reservoir the outflowing
water carries death and destruction to everything in its path.

If the leak is near the surface, as at _a_ (Fig. 155), the water
issues as a feeble stream, because the pressure against the sides at
that level is due solely to the relatively small height of water
above _a_ (Section 195). If the leak is lower, as at _b_, the issuing
stream is stronger and swifter, because at that level the outward
pressure is much greater than at _a_, the increase being due to the
fact that the height of the water above _b_ is greater than that above
_a_. If the leak is quite low, as at _c_, the issuing st