(199.)

The great uniformity of resistance produced by the traction of carriages upon a railway is such as to render the application of steam power to that purpose extremely advantageous. So far as this resistance depends on mechanical defects, it is probably rendered as uniform as is practicable, and in proportion to the quantity of load carried is reduced to as small an amount as it is likely to attain under any practicable circumstances. Until a recent period this resistance was ascribed altogether, or nearly so, to mechanical causes. The inequalities of the road-surface, the friction of the axles of the wheels in their bearings, and the various sources of resistance due to the machinery of the engine, being the principal of these resistances, were for the most part independent of the speed with which the train was moved; and it was accordingly assumed in all calculations respecting the power of locomotive engines that the resistance would be practically the same whatever might be the speed of the train. It had been well understood that so far as the atmosphere might offer resistance to the moving power this would be dependent on the speed, and would increase in a very high ratio with the speed; but it was considered that the part of the resistance due to this cause formed a fraction of the whole amount so insignificant that it might be fairly disregarded in practice, or considered as a part of the actual computed resistance taken at an average speed.

It has been, until a late period, accordingly assumed that the total amount of resistance to railway trains which the locomotive engines have had to overcome was about the two hundred and fiftieth part of the gross weight of the load drawn: some engineers estimated it at a two hundred and twentieth; others at a two hundred and fiftieth; others at a three hundred and thirtieth part of the load; and the two hundred and fiftieth part of the gross load drawn may perhaps be [Pg408] considered as a mean between these much varying estimates. What the experiments were, if any, on which these rough estimates were based, has never appeared. Each engineer formed his own valuation of this effect, but none produced the experimental grounds of their opinion. It has been said that the trains run down the engine, or that the drawing chains connecting the engine slacken in descending an inclination of sixteen feet in a mile, or 1⁄330. Numerous experiments, however, made by myself, as well as the constant experience now daily obtained on railways, show that this is a fallacious opinion, except at velocities so low as are never practised on railways.