lows it to pass
into the water, at the same time admitting a fresh supply from the
tank. The valve opens outward from the body, and hence will allow of
the exit of air but not of the entrance of water. When the diver
ceases work and desires to rise to the surface, he signals and is
drawn up by a rope attached to the suit.

192. Combination of Pumps. In many cases the combined use of both
exhaust and compression pumps is necessary to secure the desired
result; as, for example, in pneumatic dispatch tubes. These are
employed in the transportation of letters and small packages from
building to building or between parts of the same building. A pump
removes air from the part of the tube ahead of the package, and thus
reduces the resistance, while a compression pump forces air into the
tube behind the package and thus drives it forward with great speed.

CHAPTER XIX

THE WATER PROBLEM OF A LARGE CITY

193. It is by no means unusual for the residents of a large city or
town to receive through the newspapers a notification that the city
water supply is running low and that economy should be exercised in
its use. The problem of supplying a large city with an abundance of
pure water is among the most difficult tasks which city officials have
to perform, and is one little understood and appreciated by the
average citizen.

Intense interest in personal and domestic affairs is natural, but
every citizen, rich or poor, should have an interest in civic affairs
as well, and there is no better or more important place to begin than
with the water supply. One of the most stirring questions in New York
to-day has to do with the construction of huge aqueducts designed to
convey to the residents of the city, water from the distant Catskill
Mountains. The growth of the population has been so phenomenally rapid
that the combined output of all available near-by sources does not
suffice to meet the increasing consumption.

Where does your city obtain its water? Does it bring it to its
reservoirs in the most economic way possible, and is there any
legitimate excuse for the scarcity of water which many communities
face in dry seasons?

194. Two Possibilities. Sometimes a city is fortunate enough to be
situated near hills and mountains through which streams flow, and in
that case the water problem is simple. In such a case all that is
necessary is to run pipes, usually underground, from the elevated
lakes or streams to the individ