MECHANICS. 



27 



mover, so also in using the elastic force 

 of the air as a mechanical agent, we 

 consider the means whereby the neces- 

 sary degree O f elasticity is imparted to 

 it, whatever those means may be, as the 

 real prime mover. We shall see here- 

 after that, for this reason, heat assumes 

 an important rank in the class of first 

 movers. 



CHAPTER VII. Of Animal Strength. 



(57.) OXE of the most obvious, and 

 therefore one of the earliest, although, 

 perhaps, the least efficient of the prime 

 movers, is animal strength. 



From our ignoranceT of the nature 

 and principle of animal life, it is evi- 

 dent that we cannot attempt to explain, 

 on scientific principles, the laws which 

 regulate animal strength : and, on the 

 other hand, owing to the very fluctuating 

 nature of this force, the various physi- 

 cal causes which produce differences in 

 its manifestations in different indivi- 

 duals, and even in the same individual, 

 at distances of time by no means great, 

 considerable difficulties obstruct the 

 investigation and development of these 

 laws by the process of actual observa- 

 tion and experiment. The whole ana- 

 logy of nature, the beauty, order, and 

 singular harmony of. all her works, 

 however, convince us that this force, like 

 every other, is regulated by fixed laws. 



To simplify our investigations, we 

 shall consider every exertion of animal 

 strength to be represented by that which 

 is necessary to carry a load or weight. 

 It is not difficult to imagine that,~ in 

 whatever way strength be used, we can 

 find a certain load carried with a cer- 

 tain speed, which may be considered as 

 an equivalent exertion. 



In estimating the exertion of animal 

 strength in this way, one law is very 

 obvious, which is, that as the load is 

 increased (all other things being the 

 same), the velocity of the animal must 

 necessarily be diminished. But then, it 

 becomes a more difficult matter to de- 

 termine in what proportion the velocity 

 should be diminished with a given in- 

 crease of load, in order that the expen- 

 diture of animal labour should be the 

 same. Different formulae have been 

 suggested, each agreeing more or less 

 with experience, and we shall here at- 

 tempt to explain in a popular way that 

 formula which seems to represent the 

 results of experiments most accurately. 

 There are two extreme cases of ani- 



mal exertion. There is a certain speed 

 at which the animal can carry no load, 

 and can barely move its own body ; let 

 this speed be called X. There is some 

 load so great that the animal can barely 

 sustain it, without being able to move 

 it ; call this load L. What is technically 

 called the useful effect depends on two 

 things the load which is borne, and the 

 speed or velocity with which it is car- 

 ried. The useful effect is, in fact, esti- 

 mated by multiplying the load by the 

 speed. This will be easily understood 

 by an example. Suppose one horse 

 carry two hundred weight six miles an 

 hour, and another carry three hundred 

 weight four miles an hour. The load 

 of the former is two, and the speed six ; 

 the product or useful effect being twelve. 

 The load of the latter is three, and the 

 speed four ; the useful effect being 

 twelve. The -propriety of considering 

 the useful effect to be equal in these 

 two cases, will appear very evidently, 

 if we consider both horses to be em=- 

 ployed in transporting weights between 

 two places, distant one mile asunder, 

 for six hours. The first horse will 

 cany in the six hours 72 hundred 

 weight between the two places, for he 

 will make thirty-six turns, travelling 

 for six hours at six miles an hour, and 

 at each turn he will carry two hundred 

 weight. The other horse will make but 

 twenty-four turns, since he travels only 

 four miles an hour ; but then, .in each 

 turn, he will carry three hundred weight ; 

 and, therefore, he will also transport 

 in the given time 72 hundred weight 

 between the two places. Thus the 

 useful effects of these horses are equal, 

 and hence the propriety of estimating 

 the useful effect by the product of the 

 numbers which express the load and 

 the speed with which that load is carried. 

 Recurring now to the load L, and the 

 speed X, it is apparent that, with the 

 load L, the useful effect is nothing, be- 

 cause there is no speed ; and again, 

 with the speed X, the useful effect is 

 nothing, because there is no load. But 

 with a load less than L there will be 

 a speed less than X; and therefore, 

 there will be an useful effect. These, 

 then, are two limiting cases, in which 

 the useful effect vanishes, approaching 

 which it diminishes, and at some point 

 between which it is a maximum. To 

 determine where this maximum lies, it 

 is .necessary that we should know in 

 what proportion- the velocity diminishes 

 as the load increases, 



