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 the tractive force is expended in overcoming the friction, which is made up partly of the friction of attrition in the axles, and partly of the rolling friction, or the obstruction to the rolling of the wheels upon the rail.  The rolling friction is very small when the surfaces are smooth, and in the case of railway carriages does not exceed 1/1000th. of the load; whereas the draught on common roads of good construction, which is chiefly made up of the rolling friction, is as much as 1/36th of the load.

494._Q._—­In reference to friction you have already stated that the friction of iron sliding upon brass, which has been oiled and then wiped dry, so that no film of oil is interposed, is about 1/11th of the pressure, but that in machines in actual operation, where there is a film of oil between the rubbing surfaces, the friction is only about one third of this amount, or 1/33d of the weight.  How then can the tractive resistance of locomotives at low speeds, which you say is entirely made up of friction, be so little as 1/500th. of the weight?