steam, instead of the jet of water under the piston. He invented
the crank for his engine, also the sun and planet motion, also the
throttle valve, also the counter to indicate the number of revolutions
the engine had performed, also the "Cut off," the steam moving the
piston by expansion when it was cut off at one-third the length of the
cylinder, and thus saving two-thirds of the steam and a more uniform
rate of speed.

41. _Question._--Give a description of the Sun and Planet method, and
why he invented it?

_Answer._--The sun and planet were two cog-wheels geared into each
other, the sun being 3 feet diameter and the planet 2 feet diameter, the
latter was keyed tightly on the bottom end of the connecting rod, and
the sun which was keyed tightly on the end of the shaft, that was to
revolve and work the machinery. But although this method did make the
machinery revolve, it was not smoothly, for when the planet wheel was at
either top or bottom of the sun wheel, the power of the engine was less
effective than it was half way in the opposite positions. This led Watt
to add a large wheel on the shaft of the sun wheel, called the fly
wheel, which equalised the rate of motion to uniformity. Watt invented
the crank for his engine, but one of his men gave the tip to an engineer
at Bristol, who forthwith took out a patent for it and forestalled Watt,
who had to invent another means--the sun and planet. But when the term
of the patent expired, Watt resumed the crank method instead of the sun
and planet, which was noisy, the wear and tear very great, and also
expensive.

42. _Question._--What other things did Watt do towards the perfection of
the steam engine?

_Answer._--He added the air pump to his engine to draw the condensed
steam and water from his separate condenser; he invented the throttle
valve and the governor, in order to sustain a uniform rate of speed in
the engine, whatever pressure of steam might be on, or variation of
work, whether heavy or light.

43. _Question._--Why is the power of the engine called horse-power?

_Answer._--Before the invention of the engine, horses were employed in
mills and mines, and other places; the number of horses employed in a
mill or mine, indicating the amount of work going on, and the necessity
of employing them, and when the steam engine came on the scene, and a
purchaser wanted, he was told that the engine was equal to so many
horses; that comparison gave the purcha