ON THE CONSERVATION OF FORCE. 333 



sails are driven by air in motion — by wind. Air at rest 

 could just as little drive a windmill as water at rest a 

 water-wheel. The driving force depends here on the 

 velocity of moving masses. 



A bullet resting in the hand is the most harmless thing 

 in the world ; by its gravity it can exert no great effect ; 

 but when fired and endowed with great velocity it drives 

 through all obstacles with the most tremendous force. 



If I lay the head of a hammer gently on a nail, neither 

 its small weight nor the pressure of my arm is quite 

 sufficient to drive the nail into wood ; but if I swing the 

 hammer and allow it to fall with great velocity, it 

 acquires a new force, which can overcome far greater 

 hindrances. 



These examples teach us that the velocity of a moving- 

 mass can act as motive force. In mechanics, velocity in 

 so far as it is motive force, and can produce work, is 

 called vis viva. The name is not well chosen ; it is too 

 apt to suggest to us the force of living beings. Also in 

 this case you will see, f*'om the instances of the hammer 

 and of the bullet, that velocity is lost as such, when it 

 produces working power. In the case of the water-mill, 

 or of the windmill, a more careful investigation of the 

 moving masses of water and air is necessary to prove that 

 part of their velocity has been lost by the work which 

 they have performed. 



The relation of velocity to working power is most 

 simply and clearly seen in a simple pendulum, such as can 

 be constructed by any weight which we suspend to a cord. 

 Let M, Fig. 43, be such a weight, of a spherical form ; A B, 

 a horizontal line drawn through the centre of the sphere ; 

 P the point at which the cord is fastened. If now I draw 

 the weight M on one side towards A, it moves in the arc 

 M a, the end of which, a, is somewhat higher than the 

 point A in the horizontal line. The weight is thereby 



