the end of the stroke, and thus obviates what would otherwise be an
injurious shock upon the machine. When the crank approaches the lowest part
of its throw, and at the same time the piston is approaching the top of the
cylinder, the motion of the crank becomes nearly horizontal, or, in other
words, the piston is only advanced through a very short distance, for any
given distance measured on the circle described by the crank pin. Since,
then, the velocity of rotation of the crank is nearly uniform, it will
follow that the piston will move very slowly as it approaches the end of
the stroke; and the piston is brought to a state of rest by this gradually
retarded motion, both at the top and the bottom of the stroke.

109. _Q._--What causes the crank to revolve at a uniform velocity?

_A._--The momentum of the machinery moved by the piston, but more
especially of the fly wheel, which by its operation redresses the unequal
pressures communicated by the crank, and compels the crank shaft to revolve
at a nearly uniform velocity. Everyone knows that a heavy wheel if put into
rapid rotation cannot be immediately stopped. At the beginning and end of
the stroke when the crank is vertical, no force of torsion can be exerted
on the crank shaft by the crank, but this force is at its maximum when the
crank is horizontal. From the vertical point, where this force is nothing,
to the horizontal point, where it is at its maximum, the force of torsion
exerted on the crank shaft is constantly varying; and the fly wheel by its
momentum redresses these irregularities, and carries the crank through that
"dead point," as it is termed, where the piston cannot impart any rotative
force.

110. _Q._--Are the configuration and structure of the steam engine, as it
left the hand of Watt, materially different from those of modern engines?