518 THE THEORY OF ENERGY AND THE LIVING WORLD.- 



ity — which is opposing his efforts, and the man exerts a force sufficient 

 to destroy the effect of the weight. This effect, which is nullified by 

 the effort of the man, would be to cause the body to fall. The man's 

 effort is in equilibrium with the weight and is equal and opposite to it. 

 It gives him the sensation of exerting force, that is to say, the action 

 which is able to produce or prevent motion. 



The muscular activity of man may be called out in another way. 

 When workmen are employed, as Oarnot has said in his essay upon 

 equilibrium and motion, it is of no consequence " to know what burdens 

 they can reasonably support," but only those which they can carry. 

 "This is the meaning attached to the word force when it is said that 

 the horse has the force of seven men. It is not meant that if the horse 

 pulls one way and seven men the other, their efforts will be in equi- 

 librium, but that in a piece of work the horse, for example, could raise 

 as much weight to a given height in a given time as seven men." Here 

 we are concerned with the second form of muscular activity, which is 

 called in mechanics work, if we do not lay particular stress on the 

 words "in a given time," and think only of employing muscular activ- 

 ity with reference solely to its final result. Mechanical work may be 

 expressed in terms of raising a weight; and it is measured by the prod- 

 uct of the force (used in the usual sense, that is, meaning the cause of 

 motion or the hindrance to motion) by the distance through which it 

 causes motion. The unit of work is the kilogram-meter, or the work 

 required to raise a weight of a kilogram to the height of a meter. 



Time does not enter into the estimation of work; for this conception 

 is entirely free from considerations of time or velocity. "The greater 

 or less rapidity with which we execute a piece of work can not serve as 

 a measure of its amount, any more than the number of years that a 

 man spends in growing rich or in ruining himself indicates the rise or 

 decline of his fortune." 



To revert to the comparison of Carnot, a farmer who employed 

 laborers only by the job, and who would care only for the quality of 

 work irrespective of the time it occupied, would be at the same point 

 of view as those who discuss the theory of mechanics. M. Bouasse, 

 whom we follow here, remarks that this idea of work is due to 

 Descartes. His predecessors, and particularly Galileo, had an entirely 

 different method of measuring mechanical activity, and the same is true 

 of his successors, the mathematicians of the eighteenth century. 

 Leibnitz and still later Jean Bernouilli were almost alone in adopting 

 this view. 



It is precisely this idea of work that constitutes the conception of 

 mechanical energy. It represents the durable effect of mechanical ac- 

 tivity independent of all the circumstances of its execution. The same 

 work may be done under very different conditions as regards the time, 

 velocity, and force applied in its accomplishment. Energy is therefore 

 the constant element in the midst of the variety of mechanical aspect. 



