524 ON DRAUGHT. 



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 proportionally increased ; it may even, aat i i 

 we have stated, be doubled or trebled ; and the whole power of the engioa 

 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 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, as 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 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 particularly the horse, although the most ancient, 

 still remain the most advantageous source of power. 



Long experience has pointed out various modes of applying animal power; 

 but it is frequently ill directed, owing to the want of an adequate knowledge 

 of the mechanical structure of the animal, and the manner in which he exerts 

 his strength. 



In the most powerful steam-engine, if too great a resistance be applied, or, 

 practically speaking, if we attempt to make it do more work than it is calcu- 

 lated for, there is an immediate loss of power, in consequence of the diminution 

 of velocity caused thereby ; and if we continue to oppose a still greater resist- 

 ance, we reach the point at which it is unable to overcome it, and it ceases to 

 produce any effect. Again, a very small obstacle may be so applied as greatly 

 to impede an engine of considerable power, or even to stop it altogether. The 

 power of an engine is limited, and resistance must always be proportioned to it; 

 and there is a proportion beyond which it is useless to go, and less than which 

 would not absorb the whole force. 



An animal is but a beautiful piece of machinery, and although perfect in its 

 construction, and wonderfully accommodating in its movements, it still like 

 the engine, has a limited power, and has its peculiar modes of action, its strong 

 and its feeble parts ; and we must well consider its structure, to be able to 

 apply the resistance in that degree, and in that manner, which shall enable it to 

 Vrodtice the greatest effect. The consideration of the comparative effects of 



