from the forge so curved that the plain edge of the tire is
concave, and the flange edge convex, while the side which is afterward to
be bent into contact with the cylindrical surface of the wheel is a plane.
In this case the addition of the diameter of two thirds of the depth of the
flange is unnecessary, for the curving of the flange edge has the effect of
increasing the real length of the bar. When the tire is thus curved, it is
only necessary to add the thickness of the hoop to the diameter, and then
to find the circumference from a table; or the same result will be obtained
by multiplying the diameter thus increased by the thickness of the hoop by
3.1416.

529. _Q._--How are the tires attached to the wheels?

_A._--The materials for wheel tires are first swaged separately, and then
welded together under the heavy hammer at the steel works; after which they
are bent to the circle, welded, and turned to certain gauges. The tire is
now heated to redness in a circular furnace; during the time it is getting
hot, the iron wheel, turned to the right diameter, is bolted down upon a
face plate or surface; the tire expands with the heat, and when at a cherry
red, it is dropped over the wheel, for which it was previously too small,
and it is also hastily bolted down to the surface plate; the whole mass is
then quickly immersed by a swing crane in a tank of water five feet deep,
and hauled up and down till nearly cold; the tires are not afterward
tempered. The tire is attached to the rim with rivets having countersunk
heads, and the wheel is then fixed on its axle.

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 passag