Mr. S. T. Preston on the Kinetic Theory of Gravitation. 309 



motion (" heat ") of the molecules of the gas passes to the 

 aether will depend on the resistance the aether offers to the 

 passage of these molecules through it. But we have shown 

 that this resistance may (on account of the high normal velo- 

 city of the aether particles) be inappreciable. Hence the rate 

 of passage of the heat from the gas to the aether will be inap- 

 preciable. This, we submit, removes the difficulty in question. 

 It is clear that, if the aether opposes no appreciable resistance 

 to the passage of a planet through it (moving at several miles 

 per second), it cannot be affected by the passage of a molecule 

 of a gas through it, which in its relatively slow rate of trans- 

 latory motion may be considered at rest compared with the 

 aether particles. The high normal velocity of the aether 

 particles is only appropriate to their minute mass. 



16. It must be apparent to any reflecting observer, that in 

 physical science we have a vast array of facts accumulated 

 through years of experiment, but a great paucity of causes \ or 

 the number of facts known is quite out of all proportion to the 

 number of causes known, these latter being replaced by more 

 or less vague and unsubstantial theories. As, therefore, we 

 have no paucity of facts as a basis to reason upon, it surely 

 cannot be too soon to make an effort to correct this anomalous 

 state of things, and to replace the above unsubstantial theories 

 by rational conceptions of the processes of nature. Clearness 

 of conception is the test of truth, and constitutes its real dignity ; 

 and theories, however elaborated, if vague, have no real dignity*. 

 Since there is nothing occult about the physical media in space, 

 in so far as they differ in no way from ordinary matter ex- 

 cepting in the mere scale or dimensions of their parts, and 

 since it is obviously just as easy to reason of matter of one 

 dimension as of another, any hesitation in entering upon this 

 course of study would be wholly uncalled for ; indeed, surely 

 there is reason for a rational interest in realizing the admirable 

 adaptation of these media in a mechanical point of view for 

 their special functions ; and the question as to the utilization 

 of the stores of motion enclosed by them to the best advantage 

 may present a problem of the highest practical interest and 

 importance f. It should be observed that these stores of 



* Vagueness, paradox, and mystery surely belong rather to those intel- 

 lects which are incapable of rising to clear and definite conceptions. 



t As an instance of the change of views on the most practical subjects 

 that the acceptance of these principles entails, we may cite the case of 

 the employment of coal, which by the recognition of the existence of the 

 stores of motion in space, becomes a mechanism or machine for deriving 

 motion. The expenditure of coal, therefore, represents the expenditure 

 of mechanism or machinery. Hence in deriving motion through coal we 

 expend a quantity of machinery proportional to the power derived. 

 Without asking the question whether it is necessary in every case, in 

 deriving motion from a source, to expend machinery proportional to the 



