thirty years before Christ. His machine was also the first known
illustration of what is now called the "turbine" principle; the
principle of _reaction_ in mechanics. [Footnote: This principle is
often a puzzle to students. There is an old story of the man who put a
bellows in his boat to make wind against the sail, and the wind did not
affect the sail, but the boat went backward in an opposite direction
from the nozzle of the bellows. There is probably no better illustration
of reaction than the "kick" of a gun, which most persons know about. The
recoil of a six-pound field piece is usually from six to twelve feet. It
can be understood by supposing a gun to be loaded with powder and an
iron rod longer than the barrel to be left on the charge. If the outer
end of this rod were then placed against a tree, and the gun were fired,
it is manifest that the gun would become the projectile, and be fired
off of the rod backward or burst. In ordinary cases the air in the bore,
and immediately outside of the muzzle, acts comparatively, and in a
measure, as the supposed rod against the tree would. It gives way, and
is elastic, but not as quickly as the force of the explosion acts, and
the gun is pushed backwards. It is the turbine principle, running into
hundreds of uses in mechanics.] He made a closed vessel from whose
opposite sides radiated two hollow arms with holes in their sides, the
holes being on opposite sides of the tubes from each other. This vessel
he mounted on an upright spindle, and put water in it and heated the
water. The steam issuing from the holes in the arms drove them backward.
The principle of the action of Hero's machine has been accepted for two
thousand years, though never in a steam-engine. It exists under all
circumstances similar to his. In water, in the turbine wheel, it has
been made most efficacious. The power applied now for the harnessing of
Niagara for the purpose of sending electric currents hundreds of miles
is the turbine wheel.