A.—­That is the object of it, and it accomplishes its object in a very perfect manner, as it gradually arrests the velocity of the piston towards 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?

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?