of railroads; and consequently that the same power acting on a railroad, will always draw or impel twelve times the load which it can transport on a common road. But I am decidedly of opinion that a steam coach or carriage cannot be used on turnpike roads in the winter season, more particularly after a sharp frost, on a new road; and the repairs of the old road will cause that resistance at all times, that will render the attempt useless.
Having noticed the different kinds of roads over which inland transit is effected. I shall now consider the powers of traction, or the motive forces which are used on these roads. These are at present either that of horses or steam-engines.
The law which regulates the expenditure of animal strength in labour, has never yet been accurately ascertained by observation; nevertheless, there are certain general facts known respecting it, which, though not capable of being reduced to a mathematical expression, are yet sufficiently defined to lead to useful conclusions. In all cases where a horse is used as the means of transit, he must, besides the load which he bears, move the weight of his own body, and a great portion of his strength is thus employed. This portion is found to increase at a rapid rate with the velocity, so that as the speed of his motion increases, the quantity of power which he can spare to his load is as rapidly diminished. In fact, between the load which he bears, and the speed with which he is capable of moving it, there is a certain relation, which, if it could be ascertained exactly, and expressed mathematically, would give the whole theory of animal power considered as a mechanical agent. There are two obvious limiting states, between which, at some intermediate point, the effect of the horse's power is
