in the Theory of Gravitation. 453 



stone under a new form, and when it returns to the earth the 

 energy thus stored up is given back in the form of motion as 

 before. To express the matter in more precise terms — when the 

 stone is thrown up, the vis viva becomes less and less, and when 

 it reaches the turning-point it is zero. But at this point the 

 whole energy as a cause of motion is stored up, the vis viva is 

 transformed into tension, actual or kinetic energy into potential 

 energy. 



That this is not a satisfactory explanation is evident. For 

 when the stone is thrown upwards, the motion imparted gradu- 

 ally disappears. It is not converted into attraction, for the at- 

 traction, instead of being augmented by the loss of motion, is 

 found to decrease also. Hence experience shows that, according 

 to this theory, both the kinetic and the potential energy decrease 

 as the stone rises. 



No truth in physical science is now better established than 

 that force is indestructible. If it ceases to exist under one form, 

 it is because it has assumed some other form. Any conclusion 

 which stands in opposition to this principle must be abandoned, 

 whatever may be its claims for reception on other grounds. 

 When, for example, we observe the loaded piston rising under 

 the pressure of the steam, we at once conclude that the energy 

 being manifested as mechanical work existed the instant before 

 under the form of heat, and that the heat in turn existed pre- 

 viously as chemical affinity in the coal and the oxygen of the air. 

 And again, the potential energy of the coal existed in some 

 former age as sun-rays. 



When we observe mechanical work performed by heat^ or by 

 electricity, or by magnetism, &c, we at once infer that there 

 must have been a consumption of these forces corresponding to 

 the amount of work performed ; but, strange to say, although 

 we are continually witnessing the mechanical effects produced by 

 gravity, yet we are most reluctant to admit that the mechanical 

 force manifested previously existed as gravity. When a stone, 

 for example, falls to the ground, and by the concussion generates, 

 say, 100 foot-pounds of energy in the form of heat, it is at once 

 admitted that the 100 foot-pounds of energy appearing as heat 

 was derived from the mutual attraction of the earth and stone. 

 But how few will admit that there has been a consumption of 

 gravity ; and yet it is self-evident that if the total amount of the 

 earth's gravity be as great after the stone has reached the ground 

 as before it commenced its descent, then there must have been a 

 creation of power. The principle of conservation necessitates us 

 to conclude that when heat, vis viva, or work of any kind is pro- 

 duced by gravity, what we gain of actual energy in the form of 

 vis viva &c. we must lose of potential energy in the form of gra- 



