aken away, and yet the engine will
not fall down, but will be kept up by the support which the tender affords;
and the arguments hitherto paraded against the four-wheeled engines are, so
far as regards the question of safety, nothing more than arguments against
the existence of the suggested connection. It is no doubt the fact, that
locomotive engines are now becoming too heavy to be capable of being borne
on four wheels at high speeds without injury to the rails; but the
objection of damage to the rails applies with at least equal force to most
of the six-wheeled engines hitherto constructed, as in those engines the
engineer has the power of putting nearly all the weight upon the driving
wheels; and if the rail be wet or greasy, there is a great temptation to
increase the bite of those wheels by screwing them down more firmly upon
the rails. A greater strain is thus thrown upon the rail than can exist in
the case of any equally heavy four-wheeled engine; and the engine is made
very unsafe, as a pitching motion will inevitably be induced at high
speeds, when an engine is thus poised upon the central driving wheels, and
there will also be more of the rocking or sinuous motion. Locomotives,
however, intended to achieve high speeds or to draw heavy loads, are now
generally made with eight wheels, and in some cases the driving wheels are
placed at the end of the engine instead of in the middle.

508. _Q._--As the question of the locomotive boiler has been already
disposed of in discussing the question of boilers in general, it now only
remains to inquire into the subject of the engine, and we may commence with
the cylinders. Will you state the arrangement and construction of the
cylinders of a locomotive and their connections?

_A._--The cylinders are placed in the same horizontal plane as the axle of
the driving wheels, and the connecting rod which is attached to the piston
rod engages either a crank in the driving axle or a pin in the driving
wheel, according as the cylinders are inside or outside of the framework.
The cylinders are generally made an inch longer than the stroke, or there
is half an inch of clearance at each end of the cylinder, to permit the
springs of the vehicle to act without causing the piston to strike the top
or bottom of the cylinder. The thickness of metal of the cylinder ends is
usually about a third more than the thickness of the cylinder itself, and
both ends are generally made removable. The pr