530. Q.—­Is it necessary to have the whole tire of steel?

A.—­It is not indispensable that the whole tire should be of steel; but a dovetail groove, turned out of the tire at the place where it bears most on the rail, and fitted with a band of steel, will suffice.  This band may be put in in pieces, and the expedient appears to be the best way of repairing a worn tire; but particular care must be taken to attach these pieces very securely to the tire by rivets, else in the rapid revolution of the wheel the steel may be thrown out by the centrifugal force.  In aid of such attachment the steel, after being introduced, is well hammered, which expands it sideways until it fills the dovetail groove.

531. Q.—­Is any arrangement adopted to facilitate the passage of the locomotive round curves?

A.—­The tire is turned somewhat conical, to facilitate the passage 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]