time the inventor of the "throttle," or choke valve, by which he
regulated the supply of steam to the piston. It seems a strange thing
that up to this time, about 1767, an engine in actual use was started by
getting up steam enough to make it go, and waiting for it to begin, and
stopped by putting out the fire.

Then he invented the "governor," a contrivance that has scarcely changed
in form, and not at all in action, since it was first used, and is one
of the few instances of a machine perfect in the beginning. Two balls
hang on two rods on each side of an upright shaft, to which the rods are
hinged. The shaft is rotated by the engine, and the faster it turns the
more the two balls stand out from it. The slower it turns the more they
hang down toward it. Any one can illustrate this by whirling in his
hands a half-open umbrella. There is a connection between the movement
of these balls and the throttle; as they swing out more they close it,
as they fall closer to the shaft they open it. The engine will therefore
regulate its own speed with reference to the work it has to do from
moment to moment.

[Illustration: THE GOVERNOR.]

Through all these changes the original idea remained of a vacuum at the
end of every stroke, of indispensable assistance from atmospheric
pressure, of a careful use of the direct expansive power of steam, and
of the avoidance of the high pressures and the actual power of which
steam is now known to be safely capable. [Footnote: In a reputable
school "philosophy" printed in 1880, thus: "In some engines" (describing
the modern high-pressure engine, universal in most land service) "the
apparatus for condensing steam alternately above and below the piston is
dispensed with, and the steam, after it has moved the piston from one