e of the
engine round curves--the diameter of the outer wheel being virtually
increased by the centrifugal force of the engine, and that of the inner
wheel being correspondingly diminished, whereby the curve is passed without
the resistance which would otherwise arise from the inequality of the
spaces passed over by wheels of the same diameter fixed upon the same axle.
The rails, moreover, are not set quite upright, but are slightly inclined
inward, in consequence of which the wheels must be either conical or
slightly dished, to bear fairly upon the rails. One benefit of inclining
the rails in this way, and coning the tires, is that the flange of the
wheels is less liable to bear against the sides of the rail, and with the
same view the flanges of all the wheels are made with large fillets in the
corners. Wheels have been placed loose upon the axle, but they have less
stability, and are not now much used. Nevertheless this plan appears to be
a good one if properly worked out.

532. _Q._--Are any precautions taken to prevent engines from being thrown
off the rails by obstructions left upon the line?

_A_.--In most engines a bar is strongly attached to the front of the
carriage on each side, and projects perpendicularly downward to within a
short distance of the rail, to clear away stones or other obstructions that
might occasion accidents if the engine ran over them.

CHAPTER IX.

STEAM NAVIGATION.

* * * * *

RESISTANCE OF VESSELS IN WATER.

533. _Q._--How do you determine the resistance encountered by a vessel
moving in water?

_A._--The resistance experienced by vessels moving in water varies as the
square of the velocity of their motion, or nearly so; and the power
necessary to impart an increased velocity varies nearly as the cube of such
increased velocity. To double the velocity of a steam vessel, therefore,
will require four times the amount of tractive force, and as that
quadrupled force must act through twice the distance in the same time, an
engine capable of exerting eight times the original power will be
required.[1]

534. _Q._--In the case of a board moving in water in the manner of a paddle
float, or in the case of moving water impinging on a stationary board, what
will be the pressure produced by the impact?

_A._--The pressure produced upon a flat board, by striking water at right
angles to the surface of the board, will be equal to the weight of a column