408 ON DRAUGHT. 



riage, as now constructed, the force of traction is not much more than ^^-^ 

 or ^i^ oi" the 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-con- 

 structed carriage cannot be estimated in practice, at less, even when the 

 roads are in good repair, than -^-^ ; the engine, according to the construc- 

 tion of the best locomotive engines now in use, will weigh, with its car- 

 riage and fuel, at least one-half ton, or 11201bs. per horse power, and -^-^ 

 of 1120 is nearly 451bs., which 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 there- 

 fore 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 

 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 the public, unless it 

 be in cases where other means may fail to produce some particular effect 

 which may be required ; if, for instance, a considerable velocity is neces- 

 sary, 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 prin- 

 cipal object, as in the one now under consideration, and where, conse- 

 quently, little momentum is acquired, and frequent though slight obstruc- 

 tions occur, as on an ordinary road, an animal appears to possess 

 decided advantages. He adapts himself admirably to the work, increas- 

 ing 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 over- 

 come. 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 mo- 

 mentum 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 DOOlbs., and that a stone or rut 

 suddenly causes a resistance, which it would require 600 or SOOlbs. 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- 

 ing Qver 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 



