necting rods attach themselves to cranks in the driving axle.

506. _Q._--Whether are inside or outside cylinder engines to be preferred?

_A._--A diversity of opinion obtains as to the relative merits of outside
and inside cylinders. The chief objection to outside cylinders is, that
they occasion a sinuous motion in the engine which is apt to send the train
off the rails; but this action may be made less perceptible or be remedied
altogether, by placing a weight upon one side of the wheels, the momentum
of which will just balance the momentum of the piston and its connections.
The sinuous or rocking motion of locomotives is traceable to the arrested
momentum of the piston and its attachments at every stroke of the engine,
and the effect of the pressure thus created will be more operative in
inducing oscillation the farther it is exerted from the central line of the
engine. If both cylinders were set at right angles in the centre of the
carriage, and the pistons were both attached to a central crank, there
would be no oscillation produced; or the same effect would be realized by
placing one cylinder in the centre of the carriage, and two at the sides--
the pistons of the side cylinders moving simultaneously: but it is
impossible to couple the piston of an upright cylinder direct to the axle
of a locomotive, without causing the springs to work up and down with every
stroke of the engine: and the use of three cylinders, though adopted in
some of Stephenson's engines, involves too much complication to be a
beneficial innovation.

507. _Q._--Whether are four-wheeled or six-wheeled engines preferable?

_A._--Much controversial ingenuity has been expended upon the question of
the relative merits of the four and six-wheeled engines; one party
maintaining that four-wheeled engines are most unsafe, and the other that
six-wheeled engines are unmechanical, and are more likely to occasion
accidents. The four-wheeled engines, however, appear to have been charged
with faults that do not really attach to them when properly constructed;
for it by no means follows that if the axle of a four-wheeled engine
breaks, or even altogether comes away, that the engine must fall down or
run off the line; inasmuch as, if the engine be properly coupled with the
tender, it has the tender to sustain it. It is obvious enough, that such a
connection may be made between the tender and the engine, that either the
fore or hind axle of the engine may be t