270 Dynamics. 



be the value derived from this experiment for f + 1) mgt, 

 we shall have, 



neglecting m t>, when i? is small compared with g , This equa- 

 tion will enable us to judge for any other supposed value of 



, whether the force of a man will be sufficient to move the 

 weight m during the proposed time t. 



389. In all that we have now said, we have regarded' the 

 agent as acting immediately upon the weight, and as deriving 

 no advantage from local circumstances and the nature of the 

 machine. We may often rely upon a much greater effect than 

 the particular considerations now presented would lead us to 

 expect. For instance, in the use of the pulley a man may add 

 to his own proper force the weight of his body, or a large part 

 of it. There are, moreover, many other circumstances of which 

 he may avail himself, and other machines which admit of similar 

 expedients. Frequently the motion is not continued, but takes 

 place by starts, as in the pulley ; and if there is a loss on this 

 account, there is also this advantage, that the agent by intervals 

 of rest is capable of exerting the same action for a longer time. 

 We shall not dwell upon these details which it will be always 

 easy to take into the account after all that has been said, espec- 

 ially if we proceed according to experiments in which care has 

 been taken to distinguish the several causes on which the action 

 of the moving force depends, and to note what belongs to each. 



It is commonly said that a man can continue during about 

 eight hours, to exert an effort equal to 25i b . It will be seen from 

 what precedes, that such a statement does not sufficiently deter- 

 mine the value of the force in question ; besides, it is necessary, 

 as we shall soon undertake to show, to have regard to the velocity 

 with which the man acts ; it is no less necessary to consider also 

 the manner in which the action is applied, and many other cir- 

 cumstances which we cannot now stop to enumerate. It is prop- 

 er, when circumstances vary, to proceed in our calculations upon 

 new experiments made with reference to these circumstances. 



