(99.) The comparison of steam-transport with the transport by horses, even when working on a railway, exhibits the advantage of this new power in a most striking point of view. To comprehend these advantages fully, it will be necessary to consider the manner in which animal power is expended as a means of transport. The portion of the strength of a horse available for the purpose of a load depends on the speed of a horse's motion. To this speed there is a certain limit, at which the whole power of the horse will be necessary to move his own body, and at which, therefore, he is incapable of carrying any load; and, on the other hand, there is a certain load which the horse is barely able to support, but incapable of moving with any useful speed. Between these two limits there is a certain rate of motion at which the useful effect of the animal is greatest. In horses of the heavier class, this rate of motion may be taken on the average as that of 2 miles an hour; and in the lighter description of horses, 2-1/2 miles an hour. Beyond this speed, the load which they are capable of transporting diminishes in a very rapid ratio as the speed increases: thus, if 121 express the load which a horse is able to transport a given distance in a day, working at the rate of four miles an hour, the same horse will not be able to transport more than the load expressed by 64, _the same distance_, at 7 miles an hour; and, at 10 miles an hour, the load which he can transport will be reduced to 25. The most advantageous speed at which a horse can work being 2 miles an hour, it is found that, at this rate, working for 10 hours daily, he can transport 12 tons, on a level railway, a distance of 20 miles; so that the whole effect of a day's work may be expressed by 240 tons carried 1 mile.
But this rate of transport is inapplicable to the purposes of travelling; and therefore it becomes necessary, when horses are the moving power, to have carriages for passengers distinct from those intended for the conveyance of goods; so that the goods may be conveyed at that rate of speed at which the whole effect of the horse will be the greatest possible; while the passengers are conveyed at that speed which, whatever the cost, is indispensably necessary. The weight of an ordinary mail-coach is about two tons; and, on a tolerably level turnpike road, it travels at the rate of 10 miles an hour. At this rate, the number of horses necessary to keep it constantly at work, including the spare horses indispensably necessary to be kept at the several stages, is computed at the rate of a horse per mile. Assuming the distance between London and Birmingham at 100 miles, a mail-coach running between these two places would require 100 horses; making the journey to and from Birmingham daily. The performance, therefore, of a horse working at this rate may be estimated at 2 tons carried 2 miles per day, or 4 tons carried 1 mile in a day. The force of traction on a good turnpike road is at least 20 times its amount on a level railroad. It therefore follows, that the performance of a horse on a railroad will be 20 times the amount of its performance on a common road under similar circumstances. We may, therefore, take the performance of a horse working at 10 miles an hour, on a level railroad, at 80 tons conveyed 1 mile daily.