ON DRAUGHT. 409 



increased. Thus, if the road be slightly muddy or sandy, or newly grav- 

 elled, the draught, as we shall see more accurately laid down when we 

 come to the subject of wheeled-carriages, will be double and even treble 

 what it is on the same road when freed from dust or dirt; but the average 

 power of the horse remains nearly the same, and, practically speaking, 

 equal under both circumstances; that is to say, that the power necessary 

 to move the weight of the horse's body, which forms no inconsiderable por 

 tion of his whole power, is not materially increased by a state of road 

 which will even treble the draught of the carriage; consequently, the 

 excess, or available portion of his power, remains unimpaired, and the full 

 benefit of it, as well as of any increased exertions of the animal, is felt and 

 is applied solely to dragging the load. 



Not so with a locomotive steam-engine, because beyond the power neces- 

 sary to perform the work of dragging the load, an additional power must be 

 provided to move the engine itself. In other words, if an engine of ten- 

 horse power be capable of dragging a certain load, the weight of this 

 engine forming a portion of the load to be moved, a corresponding portion 

 of the power is unprofitably absorbed in moving it, and the excess, or 

 remaining power, is alone available for useful purposes, and can alone be 

 compared to the animal or horse power. Now, if the draught is augmented, 

 as we have just supposed, by any sand, dirt, or roughness of the road, or 

 any other impediment, the force required to move the useless weight 

 (of the engine) is proportionably increased ; it may even, as we have 

 stated, be doubled or trebled : and the whole power of the engine remaining 

 the same, the surplus or remaining portion is considerably diminished, and 

 that at the very moment when, as before stated, it produces only one-half 

 or one-third the effect. 



Moreover, if at any part of the road a power equal to twenty horses is 

 required, the engine, as regards its construction, must be a twenty-horse 

 engine. It is erroneous to suppose that a steam-engine, because it is a high- 

 pressure engine, can therefore, as occasion requires, be worked for any 

 length of time beyond its nominal power, by merely raising the steam. 

 Every part of a machine is calculated and arranged for a certain pressure 

 and corresponding power, and that is the real power of it. It is optional 

 to work at or below that power, but, if below, it will be to a disadvantage, 

 and the bulk and weight of the machine will be as great as if it were 

 always worked to its full extent, and both have to be carried over all those 

 parts of the road where a far less power would be sufficient. The velocity 

 of the carriage might indeed be increased, while travelling on the good and 

 level portion of the road, but these alternations in the speed and power can- 

 not be effected without a considerable degree of complexity, weight, and 

 expense in the machinery, and as we are confining ourselves to the con- 

 sideration of the case where velocity is not required, and might even be an 

 inconvenience, the excess of power will be wasted. 



These objections to the use of mechanical power, in certain cases, are 

 pointed out, not as being insurmountable obstacles to the use of machinery, 

 but as serious difficulties, which, in practice, have not yet been overcome. 

 In fact, there is not at present any practical substitute for horse power on 

 common roads, and as far as the public is concerned nothing has yet been 

 done. We, therefore, must consider them as objections remaining to be 

 overcome ; and we are compelled to draw the conclusion, that, at the present 

 moment, animal power (always confining ourselves to the question of the 

 economical transport of heavy goods upon common roads) is superior to 

 any mechanical agent, and that beasts of draught, and particulaa-ly th« 

 Dd 



