266 KINETIC THEORIES OF GRAVITATION. 



through the very center of the space occupied by our earth only two 

 weeks previously without any perturbation or consciousness of increased 

 tension in this mystic " space.'" To affirm that the projected stone 

 transfers its vis viva to space without suggesting any conceivable method 

 by which such transfer could physically be effected, is not to proffer an 

 hypothesis. To affirm that the falling stone receives from the illimitable 

 ocean of space its just supply of vis viva — whether required at the in- 

 stant or ten years afterward — is to bid us hope that the steam exhausted 

 from the cylinder into the atmosphere may be induced to return to its 

 duty when needed, in order to justify our faith in its conservation. 



Nine years later, in a communication read before the British Asso- 

 ciation at Glasgow, September, 187G, " On the Transformation of 

 Gravity," Mr. Croll repeats very much the line of argument just re- 

 ferred to, showing that the interval had not served to remove his diffi- 

 culties. He commences his memoir with the query: " Is gravity con- 

 vertible into other forms of energy? Can gravity be converted into 

 heat, electricity, magnetism, etc.? or can those forms of energy be con- 

 verted into gravity?" It might be supposed that the question would as 

 soon occur to the physicist. Can the flight of the arrow be converted 

 into elastic tension"? The answer to his inquiry is directly involved in 

 the fifth and sixth propositions. Mr. Croll, however, gives a different 

 answer. "It may be true that gravity cannot be directly transformed 

 into heat, electricity, magnetism, chemical affinity, etc., nor these forms 

 ■directly transformed into gravity; but nevertheless, the thing may be 

 done indirectly. ... If the electricity produced by the descent of the 

 water be gravity transformed into electricity, then the ascent of the 

 water i^roduced by electricity must be electricity transformed into grav- 

 ity ; for it is the same process merely reversed."* The alternatives are 

 doubtless equally correct. 



" If gravity be correlated to other forms of energy, it must like them 

 come under the great principle of conservation. But here we enter 

 upon debatable ground. It is admitted that gravity can perform me- 

 chanical work, and the mechanical work can be converted into other 

 forms of energy. Here we have correlations; but it is generally denied 

 that there is a decrease or loss of gravity resulting from such trans- 

 formations. But this appears to me to be a virtual denial of the prin- 

 ci[)ie of conservation. . . . The reasons which appear to have led to 

 this opinion are I think, mainly the two following : 1. It has been as- 

 sumed [!J that the weight of a body is not affected by the work which 

 it performs. 2. The force by which bodies are drawn toward each other 

 does not diminish as they approach, but on the contrary increases. 

 . . . May not a stone when in the act of falling be acted upon by 

 gravity with less force at any given moment than it would be were the 

 stone at rest at that instant ? The point has never yet been determined 



* Phil. Mag., October, 187G, vol. ii, pp.241, 242. 



