ON THE CONSERVATION OF FORCE. 321 



son eliminate anything in which activity of intelligence 

 comes into play. The latter is also capable of the hard 

 and intense -work of thinking, which tries a man just as 

 muscular exertion does. But whatever of the actions of 

 intelligence is met with in the work of machines, of course 

 is due to the mind of the constructor and cannot be 

 assigned to the instrument at work. 



Now, the external work of man is of the most varied 

 kind as regards the force or ease, the form and rapidity, 

 of the motions used on it, and the kind of work produced. 

 But both the arm of the blacksmith who delivers his 

 powerful blows with the heavy hammer, and that of the 

 violinist who produces the most delicate variations in 

 sound, and the hand of the lace-maker who works with 

 threads so fine that they are on the verge of the invisible, 

 all these acquire the force which moves them in the same 

 manner and by the same organs, namely, the muscles of 

 the arm. An arm the muscles of which are lamed is in- 

 capable of doing any work ; the moving force of the 

 muscle must be at work in it, and these must obey the 

 nerves, which bring to them orders from the brain. 

 That member is then capable of the greatest variety of 

 motions ; it can compel the most varied instruments to 

 execute the most diverse tasks. 



Just so is it with machines : they are used for the most 

 diversified arrangements. We produce by their agency 

 an infinite variety of movements, with the most various 

 degrees of force and rapidity, from powerful steam- 

 hammers and rolling-mills, where gigantic masses of iron 

 are cut and shaped like butter, to spinning and weaving- 

 frames, the work of which rivals that of the spider. 

 Modern mechanism has the richest choice of means of 

 transferring the motion of one set of rolling wheels to 

 another with greater or less velocity ; of changing the 

 rotating motion of wheels into the up-and-down motion 



