August, I9II 
procedure is continued until the air which formerly filled 
the tank occupies only one-half the space, then this cushion 
of air will exert the very strong pressure of about fifteen 
pounds per square inch on the water. ‘This is equivalent to 
using an elevated tank with the water surface at the height 
of 33.9 feet. If we continue to pump water in until the air 
occupies only one-fourth of its former space, then we have 
AMERICAN HOMES AND GARDENS 
237 
exert its pressure impartially in all directions; we desire its 
pressure on the water; we get it there, and everywhere else 
besides. 
As the compression goes on, consequently, the tank 
will leak, unless it has been manufactured by skilled makers 
fully alive to the necessities. It is said to be a most difficult 
thing to construct an air-tight tank that will not leak. It 
Fig. 5—Brick and stone tower with tank 
in the enclosure at top 
done the equivalent of lifting the water to the height of 
101.7 feet. Now, all this is very simple in principle. We 
can, in fact, get any reasonable pressure desired by pumping 
in water and compressing the trapped air. A little thought 
will, however, show us that perhaps the practical carrying 
out of the principle will not be as easy as it looks. There 
are two principal difficulties: The air as it is compressed will 
Fig. 8B—A stucco tower with the tank surrounded by a balcony 
Fig. 6—The tank and windmill is built on 
top of the tower 
Fig. 7—A wooden tower with windmill at 
top and tank enclosed underneath 
seems that a boiler that will not leak steam under pressure 
may nevertheless leak air under pressure. ‘The particles of 
air are, apparently, smaller in size, or are otherwise com- 
petent to push their way more successfully through minute 
cracks. The air-tank must, accordingly, be better than a 
steam boiler needs to be. One of the principal builders of 
pneumatic apparatus states that the organization of the best 
- : — oes 
a. |, a ~ tae goa satis 
Fig. 9—A tower designed in the form of a windmill 
