ON DRAUGHT. 409 



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

 gravelled, 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 

 necessary 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 aug- 

 mented, 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 proportionally increased ; it may even, as we have 

 stated, be doubled or trebled ; and the whole power of the engine remain- 

 ing 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 20-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 

 cannot be effected without a considerable degree of complexity, weight, 

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

 consideration 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 pre- 

 sent 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 particularly the 



