336 ON THE CONSERVATION OF FORCE. 



and of friction. Neither does it increase, but it continually 

 changes the form of its manifestation. 



Let us now pass to other mechanical forces, those 

 of elastic bodies. Instead of the weights which drive 

 our clocks, we find in time-pieces and in watches, steel 

 springs which are coiled in winding up the clock, and 

 are uncoiled by the working of the clock. To coil up the 

 spring we consume the force of the arm ; this has to 

 overcome the resisting elastic force of the spring as we 

 wind it up, just as in the clock we have to overcome the 

 force of gravity which the weight exerts. The coiled 

 spring can, however, perform work ; it gradually expends 

 this acquired capability in driving the clockwork. 



If I stretch a crossbow and afterwards let it go, the 

 stretched string moves the arrow ; it imparts to it force 

 in the form of velocity. To stretch the cord my arm 

 must work for a few seconds ; this work is imparted 

 to the arrow at the moment it is shot off. Thus the 

 crossbow concentrates into an extremely short time 

 the entire work which the arm had communicated in the 

 operation of stretching; the clock, on the contrary, 

 spreads it over one or several days. In both cases no 

 work is produced which my arm did not originally impart 

 to the instrument, it is only expended more conveniently. 



The case is somewhat different if by any other natural 

 process I can place an elastic body in a state of tension 

 witliout having to exert my arm. This is possible and 

 is most easily observed in the case of gases. 



If, for instance, I discharge a fire-arm loaded ^^ith 

 gunpowder, the greater part of the mass of the powder is 

 converted into gases at a very high temperature, which 

 have a powerful tendency to expand, and can only be 

 retained in the narrow space in which they are formed, 

 by the exercise of the most powerful pressure. In 

 expanding with enormous force they propel the bullet, 



