408 ON DRAUGHT. 



riage as now constructed, the force of contraction is not nnuch mere than 

 Tso "'" 2^i(r "'" ^''^ weight moved; consequently, the power necessary to 

 move the engine itself is not very considerable. On a road, however, this 

 proportion is materially altered; here the average force required to move a 

 well-constructed carriage cannot be estimated in practice, at less, even 

 when the roads are in good repair, than ^' ^ ; the engine, according to the 

 construction of the best locomotive engines now in use, will weigh, with 

 its carriage and fuel, at least one-lialf ton, or 1120 lbs. per hoise power, and 

 ^L of 1120 is nearly 45 lbs., wiiich we have to deduct from the gross 

 power of the engine, and which leaves only 121|- lbs. as the available 

 power. The proportional 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 if mechanical power was found as convenient and 

 as applicable in practice as horse power, still no great economy can be 

 expected from the employment, upon common roads, of small locomotive 

 engines, such as the best of those now in use, and known to tlie public, 

 unless it be in cases where other means may fail to produce some particu- 

 lar 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 princi- 

 pal 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 imme- 

 diately appreciated. 



By mechanical power, such as a steam-engine affords, these advantages 

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

 tum can be acquired; and the carriage itself, not being in rapid motion, 

 and the necessary economy in weight precluding the use of a fly-wheel, 

 any 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 000 or 800 lbs. to overcome, a 

 case by no means rare even on tolerable roads; if the impetus or momen- 

 tum 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 remain- 

 ing over and above that necessary to carry his own body ; and that in all 

 ordiqary cases he is able to maintain and continue nearly the same excr- 

 tions, although the comparative draught of the carriage be considerably 



