Dr. J. Croll on the Transformation of Gravity. 251 



2nd. In every possible case the amount of transformation 

 is limited by the extent of the space condition available for 

 the forces to act. We have seen that in the steam-engine, 

 for example, the amount of energy in the form of heat that 

 can be converted into mechanical work would be limited by 

 the length of the cylinder, were it not that the stroke of the 

 piston is reversed ; for when the piston reached the end of 

 the cylinder no further transformation of energy could take 

 place. But the reversal of the motion of the piston affords 

 an unlimited amount of space condition ; consequently, so 

 far as space is concerned, there is no limit to the amount of 

 heat which may be converted into work in a steam-engine. 

 The same thing is also true of the molecules of the steam 

 itself. They could perform no work on the piston unless they 

 had space to allow of motion. But as their motion, like that 

 of the piston, is to and fro, there is no limit, so far as space 

 is concerned, to the amount of energy which they may lose 

 in work. 



But gravity is a force which cannot act to and fro. The 

 amount of energy which gravity can lose is limited by the 

 distance between the gravitating bodies. When the space 

 between them is traversed and the bodies are in contact, there 

 is then no space condition, and further transformation of 

 energy into work is impossible. 



Energy of Position. — What is meant by energy of posi- 

 tion must simply be that a body is in such a position that 

 force may be converted into kinetic energy or energy of 

 motion. It means that the force is in a position, so far as 

 the space condition is concerned, of being transformed into 

 kinetic energy. We do not speak of the piston of the steam- 

 engine being possessed of energy of position. We have no 

 occasion to do so, simply because it is always in possession of 

 energy of position. But we speak of the weight when raised 

 and the bow when bent as possessing energy of position ; for 

 it is only when they are in this condition that force can be con- 

 verted into motion. The generally received meaning, however, 

 of these terms is not only erroneous and misleading, but hides 

 a most important physical truth. For when it is said that a 

 body possesses energy of position, it is not meant simply that 

 the body, in regard to space, is in a position to allow of force 

 being transformed into energy of motion ; for it is not sup- 

 posed that force is transformed at all, the only thing changing 

 being simply the position of the body. When a stone, dropped 

 from a height, is being pulled to the ground by the force of 

 gravity, it is supposed that there is no loss or consumption of 

 gravity ; the only thing lost which can possibly be regarded 



