Chapter LIV: Section 90: ; Turbines, Water-pressure Engines, &c., Section 93

=Descending weights= must first of course be raised, absorbing as much
power in raising as they give out in falling, neglecting friction.
Clockwork; water; or compression of a spring (see Section 80);
multiplying pulleys (see Section 42), are the apparatus employed to
utilise this form of energy.

=Wave motion= is too uncertain and erratic to be a practicable source
of power. Rocking air-compressing chambers, rocking pumps, &c., have
obtained some small measure of success.

=Wind,= windmills. See Section 95.

=Expansion of air and gases.= Ascending currents of hot air from a
fire are used to drive a light screw motor, fan, &c. Hot-air engines,
see Ryder’s patent and numerous others, which depend upon alternate
expansion and contraction of air by heating and cooling. Air
compressed in an accumulator or reservoir is employed to give motion
to multiplying pulleys or an air engine.

=Expansion of liquids,= other than water (by heat), into the gaseous
form. Engines in which the fuel is burnt under pressure and the total
products of combustion employed (with or without steam) to drive a
motor.

=Steam= is in reality one of the last-mentioned sources of power; it
is employed by direct pressure on a piston or ram (see Section 32); or
to produce direct rotary motion (see Section 75); also in the jet
pump, No. 801; or injector (see Section 45); or by direct pressure on
a body of water contained in a closed vessel, as in the pulsometer,
steam accumulator, &c.

=Explosives= are substances which, by application of flame, heat,
percussion, &c., suddenly assume the gaseous form, thus increasing
their bulk many hundred times, usually in a small fraction of a second
of time. A second class comprise explosive mixtures of gases, such as
hydrogen, and oxygen, carburetted hydrogen, and air. Some attempts
have been made to employ explosive substances to drive engines in
various ways, but with no permanent success. The second class of
explosive mixtures of gases are largely employed in the gas engine,
petroleum engine, and their varieties.

=Fuels, hydrocarbons,= &c., are employed to evaporate water into
steam; to expand air or other gases, or convert liquids into gases;
and also by vaporisation to supply gas for use in some forms of gas
engine.