532 ON DKAUGHT. 



is nearly 45 lbs., wLicla we have to deduct from the gross power of the 

 engine, and which leaves only 121f lbs. as the available power The pro- 

 poi^ional expense of the horse and the steam-engine is now therefore about 

 as 115 to 100, and this without taking into account the causes of increased 

 expenditure already alluded to as regards the prime cost, the repairs and 

 the consumption of fuel. From these calculations it would appear 'that 

 even il mechanical power was found as convenient and applicable in prac- 

 tice as horse-power, stUl no great economy can be expected from the em- 

 ployment, upon common roads, of small locomotive engines, such as the 

 best of those now m use, and kno^vn 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 bodV,' 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 advan- 

 tages on a very good road, which in fact approaches very nearly to a 

 railway. But m every case in which velocity is not a principal object as 

 m the one now under consideration, and where, consequently, little mo- 

 mentum is acquired, and frequent though sKght obstructions occur as on 

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

 adapts hunself admu-ably 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 be- 

 comes enabled to make exertions far beyond his average streno-th where 

 any impediment or obstruction is to be overcome. Indeed he appears 

 rather to increase the average effect of Ms poivms by these alternations of 

 exertion and comparative relaxation ; and when it is considered that the 

 draught will, m 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 advantao-es 

 are not easily obtained. Without great weight or rapid motion no momen- 

 tum can be acquired ; and, unless when the carriage is in very rapid mo- 

 tion 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 500 lbs., and that a stone or rut 

 suddenly causes a resistance, which it would require 800 or 1000 lbs 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 obstruc- 

 tion, 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 remain- 

 mg 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 exer- 

 tions, 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 ca,rriages, will be double and even 

 treble what it is on the same road when freed from dust and dirt • but the 

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

 speaking, equal in both circmnstances ; 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 mil even treble the draught of the carriage ; consequently, 

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



