linder be a fixed one it
should be bolted down to the sole plate by as many bolts as are employed to
attach the cylinder cover, and they should be of copper or brass, in any
situation that is not easily accessible.

491. _Q._--If the engines become loose, how do you refix them?

_A._--It is difficult to fix engines effectually which have once begun to
work in the ship, for in time the surface of the keelsons on which the
engines bear becomes worn uneven, and the engines necessarily rock upon it.
As a general rule, the bolts attaching the engines to the keelsons are too
few and of too large a diameter: it would be preferable to have smaller
bolts, and a greater number of them. In addition to the bolts going through
the keelsons or the vessel's bottom, there should be a large number of wood
screws securing the sole plate to the keelson, and a large number of bolts
securing the various parts of the engine to the sole plate. In iron
vessels, holding down bolts passing through the bottom are not expedient;
and there the engine has merely to be secured to the iron plate of the
keelsons, which are made hollow to admit of a more effectual attachment.

492. _Q._--What are the proper proportions of bolts?

_A._--In well formed bolts, the spiral groove penetrates about one twelfth
of the diameter of the cylinder round which it winds, so that the diameter
of the solid cylinder which remains is five sixths of the diameter over the
thread. If the strain to which iron may be safely subjected in machinery is
one fifteenth of its utmost strength, or 4,000 lbs. on the square inch,
then 2,180 lbs. may be sustained by a screw an inch in diameter, at the
outside of the threads. The strength of the holding down bolts may easily
be computed, when the elevating force of the piston or main centre is
known; but it is expedient very much to exceed this strength in practice,
on account of the elasticity of the keelsons, the liability to corrosion,
and other causes.

THE LOCOMOTIVE ENGINE.

493. _Q._--What is the amount of tractive force requisite to draw carriages
on railways?

_A._--Upon well formed railways with carriages of good construction, the
average tractive force required for low speeds is about 7-1/2 lbs. per ton,
or 1/300th of the load, though in some experimental cases, where particular
care was taken to obtain a favorable result, the tractive force has been
reduced as low as 1/500th of the load. At low speeds the whole of