86 Prof. Briicke on Gravitation and 



original motion will be reproduced. In this latter position no 

 other cause of motion acts upon the pendulum than its own mo- 

 ment of inertia. When the pendulum moves away from its posi- 

 tion of equilibrium, and then loses velocity, but at the same time 

 generates a cause of motion, wc express it by saying that vis viva 

 is transformed into tension* («c/?iff/ energy into iJotential ci\CY^y\). 

 When the pendulum again approaches its position of equilibrium, 

 and from the cause of motion, motion itself follows, then wc say 

 tension is transformed into vis viva [potential energy into actual 

 energy). An analogous state of things is observed throughout 

 nature, from the motion of the heavenly bodies to the motion of 

 the flame which flickers in our chimney, and in which the atoms 

 are freed from that mysterious state of tension into which they 

 have been forced by years of action of the solar rays. 



But let us return to the example of Faraday. Let us first fix 

 our attention on the mass A, which is removed from the mass B. 

 Let us suppose B to be the earth, and A a stone which is cast from 

 the earth upwards. Let the earth be supposed to be at rest, and 

 let the influence of the atmosphere and the heavenly bodies be 

 disregarded. After the velocity of the stone has sunk to zero 

 it will fall back, and on reaching the earth will have attained a 

 final velocity, the half-square of which, multiplied by the mass, 

 gives a quantity of force, which, if exerted upwards, would be 

 exactly sufficient to carry the stone to the height from which it 

 has fallen. This is a known fact, and generally the first example 

 with which the law of the conservation of foixe is illustrated. If 

 I imagine the stone to be cast higher and higher from the earth, 

 nothing is thereby changed, except that in the more distant por- 

 tions of its path the velocity is more slowly consumed, and on 

 returning is more slowly generated, than in the nearer portions : 

 the final result is always the same, only a longer time passes 

 before it arrives. Let us regard the stone at the moment when 

 its velocity is zero, that is, when it is at its greatest distance from 

 the earth. Has then the moving force, that is, the cause of 

 motion, diminished at the moment in question in the system 

 formed of Ihe stone and the earth ? By no means. It is true 

 that the stone, by an external force, may now be more easily re- 

 moved from the earth, and that it commences to fall with a 

 slower acceleration of its motion than woidd be the case if it 

 started from a point nearer to the earth ; but there is in the stone 

 a store of force accumulated, in virtue of which it attains a greater 

 final velocity the further it has been removed from the earth. 



The terms 'living force,^ 'tension force,' 'moving force,' as they 



* See Helinholtz "Oa the Conservation of Force," Sclent. Mem. 1853. 

 p. 124.— Ed. 



t Rankine's "Outlines of the Science of Energetics," Edinb. Phil. Jomu. 

 July 1855.— Ed. 



