ON DRAUGHT. 523 



employment, upon common roads, of small locomotive engines, such an the best 

 of those now in use, and known to the public, unless it is in cases where other 

 means may fail to produce some particular effect which may be required ; if, for 

 instance, a considerable velocity is necessary, the power of a horse is very nearly 

 exhausted in moving his own body, and then there can be no doubt that a 

 mechanical agent, in which power may always be exchanged for a proportional 

 velocity, will have some advantages on a very good road, which in fact approaches 

 very nearly to a railway. But in every case in which velocity is not a principal 

 object, as in the one now under consideration, and where, consequently, little 

 momentum is acquired, and frequent though slight obstructions occur, as on an 

 ordinary road, an animal appears to possess decided advantages. He adapts 

 himself admirably to the work, increasing or diminishing his efforts according to 

 the variations of the draught, resting himself, as it were, and acquiring vigour 

 where his utmost strength is not called for, and thus becomes enabled to make 

 exertions far beyond his average strength where any impediment or obstruction 

 is to be overcome. Indeed, he appears rather to increase the average effect of his 

 powers by these alternations of exertion and comparative relaxation ; and when 

 it is considered that the draught will, in an ordinary road, frequently vary in the 

 proportion of six or eight to one, and that these changes may succeed each other 

 suddenly, the importance of such an accommodating faculty will be immediately 

 appreciated. 



By mechanical power, such as a steam-engine affords, these advantages are not 

 easily obtained. Without great weight or rapid motion no momentum can be 

 acquired j and unless when the carriage is in very rapid motion, a very small 

 obstruction will check, and perhaps totally stop the machine. For instance, 

 supposing the carriage to be advancing steadily under the effect of a force of 

 traction of 5001bs., and that a stone or rut suddenly causes a resistance, which 

 it would require 800 or 10001b. to overcome, a case by no means rare even on 

 tolerable roads ; if the impetus or momentum of the mass be not sufficient to 

 carry it over this obstruction, the machine must stop until some increased power 

 be given to it. 



It is also to be remembered, that what we are accustomed, in practice, to 

 consider as the average power of a horse, is the average excess remaining over 

 and above that necessary to carry his own body ; and that in all ordinary cases 

 he is able to maintain and continue nearly the same exertions, although the 

 comparative draught of the carriage be considerably 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 in both circumstances ; that is to say, that the 

 power necessary to move the weight of the horse's body, which forms no incon- 

 siderable portion 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, a large additional power must be pro- 

 vided 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. 



