Prime Movers. 11 



in power ; and each succeeding age shall unfold wonders that 

 were not even dreamed of in the preceding. Constant efforts, 

 therefore, have been made to supersede the steam-engine, by- 

 some motive power still more convenient or economical. What 

 will be the result of similar attempts hereafter, none can say ; 

 for it would be rashness to place a limit to the discoveries of 

 science j but, hitherto, as we shall presently perceive, success 

 has not attended these efforts. On the contrary, time, labour, 

 and ingenuity have been wasted on projects which, being 

 opposed, in many instances, even by physical laws, were 

 utterly impracticable. We shall consider, in succession, the 

 various sources of motive power ; directing our attention prin- 

 cipally to those contrivances which have been intended, either 

 seriously to modify the present mode of applying steam, or 

 altogether to set it aside. 



Although the strength of animals has been used from the 

 earliest times, we have arrived at but little accuracy, and no 

 uniformity, in our modes of determining even its average amount 

 for any species; and the estimates which have been mado re- 

 garding it vary considerably. This, however, should cause no 

 surprise ; since not only do animals of the same species differ 

 in their capabilities, but the very same animal gives different 

 results, according to the nature of its employment, its intervals 

 of rest, the kind and quantity of its food, and a number of 

 other circumstances. The strength of an animal is equal to 

 " the product of its effort, its velocity, and that part of the 

 twenty-four hours during which the effort is continued/' And 

 there is, for each individual, some set of values of these quan- 

 tities which gives its maximum amount of work. An animal 

 may move so fast, as to be able to move only itself; or, from 

 being overburdened, so slowly, as to produce no useful effect. 

 The proper speed and burden lie between these extremes. An 

 animal may be employed either in carrying, in pushiug, or in 

 drawing. A man will carry, on a horizontal plane, eighty-five 

 and a~half pounds, for seven hours a-day, two feet and four-tenths 

 per second ; which is equivalent to 5,171,000 pounds carried one 

 foot. He will draw with a force of from seventy to eighty pounds 

 on the level ground ; but will push at the height of his shoulders 

 with a force of only about twenty-seven or thirty pounds. He can 

 ascend a flight of steps, unburdened, at the rate of twelve twenty- 

 fifths of a foot per second, which — supposing him to be of aver- 

 age weight — is equivalent to 1,935,000 pounds lifted one foot, or 

 only two twenty -fifths of what he could do, moving horizontally. 



Of all animals, the horse is best adapted for labour. And, 

 since he uses his weight to overcome resistance, his efforts are 

 most effectively exerted in drawing a load on a horizontal surface. 

 Watt considered that a horse could raise only 33,000 lbs. one 



