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?

_A._--There is not much difference. In modern rotative land engines, the
valves for admitting the steam to the cylinder or condenser, instead of
being clack or pot-lid valves moved by tappets on the air pump rod, are
usually sluice or sliding valves, moved by an eccentric wheel on the crank
shaft. Sometimes the beam is discarded altogether, and malleable iron is
more largely used in the construction of engines instead of the cast iron,
which formerly so largely prevailed. But upon the whole the steam engine of
the present day is substantially the engine of Watt; and he who perfectly
understands the operation of Watt's engine, will have no difficulty in
understanding the operation of any of the numerous varieties of engines
since introduced.

THE MARINE ENGINE.

111. _Q._--Will you describe the principal features of the kind of steam
engine employed for the propulsion of vessels?

_A._--Marine engines are of two kinds,--paddle engines and screw engines.
In the one case the propelling instrument is paddle wheels kept in rotation
at each side of the ship: in the other case, the propelling instrument is a
screw, consisting of two or more twisted vanes, revolving beneath the water
at the stern. Of each class of engines there are many distinct varieties.

112. _Q._--What are the principal varietie