enables him to maintain a state of equilibrium
between the upward pull of the gas and the downward pull of gravity. To
sink he lets out gas, to rise he throws out ballast; and this process
can be repeated until the ballast is exhausted. The greatest height ever
attained by aeronauts is the 7-1/4 miles, or 37,000 feet, of Messrs.
Glaisher and Coxwell on September 5, 1862. The ascent nearly cost them
their lives, for at an elevation of about 30,000 feet they were partly
paralyzed by the rarefaction of the air, and had not Mr. Coxwell been
able to pull the valve rope with his teeth and cause a descent, both
would have died from want of air.

[Illustration: FIG. 171.]

The _flying-machine_, which scientific engineers have so long been
trying to produce, will probably be quite independent of balloons, and
will depend for its ascensive powers on the action of air on oblique
surfaces. Sir Hiram Maxim's experimental air-ship embodied the
principles shown by Fig. 171. On a deck was mounted an engine, E,
extremely powerful for its weight. This drove large propellers, S S.
Large aeroplanes, of canvas stretched over light frameworks, were set up
overhead, the forward end somewhat higher than the rear. The machine was
run on rails so arranged as to prevent it rising. Unfortunately an
accident happened at the first trial and destroyed the machine.

In actual flight it would be necessary to have a vertical rudder for
altering the horizontal direction, and a horizontal "tail" for steering
up or down. The principle of an aeroplane is that of the kite, with this
difference, that, instead of moving air striking a captive body, a
moving body is propelled against more or less stationary air. The
resolution of forces is shown by the arrows as before.

Up to the present time no practical flying-machine has appeared. But
experimenters are hard at work examining the conditions which must be
fulfilled to enable man to claim the "dominion of the air."

[34] The "Romance of Modern Mechanism," p. 243

Chapter XVIII.

HYDRAULIC MACHINERY.

The siphon--The bucket pump--The force-pump--The most marvellous
pump--The blood channels--The course of the blood--The hydraulic
press--Household water-supply fittings--The ball-cock--The
water-meter--Water-supply systems--The household filter--Gas
traps--Water engines--The cream separator--The "hydro."

In the last chapter we saw that the pressure of the atmosphere is 1