32 Prof. Challis on Newton's 



(4) On the same principles, the existence of a mechanical 

 equivalent of heat is accounted for (see ' Principles &c.' 

 pp. 469 and 481). 



(5) It may plainly be demanded of this advanced depart- 

 ment of physical philosophy to give reasons for the qualities 

 of the three hypotheses on which, as before stated, the science 

 of physical astronomy rests. It affords, in fact, the following 

 reasons : — (1) The universality of gravitation is a direct con- 

 sequence of its being a mode of action of a universal medium, 

 the aether. (2) The gravitation law of the inverse square is 

 mathematically derived from the dynamical action of aethe- 

 rial undulations of large magnitude on an atom taken to be a 

 small sphere. The most complete investigation of this law 

 that I have succeeded in giving on hydrodynamical principles, 

 I consider to be that contained in arts. 31-38 of a communi- 

 cation on Attractive and Repulsive Forces, in the Phil. Mag. 

 of September 1872. (3) In art. 39 of the same communica- 

 tion, an argument embracing the squares of the velocities of 

 the aether is adduced, from which the coexistence of the trans- 

 latory effects of all forces referable to the action of aetherial 

 vibrations is inferred. Hence, after accounting for universal 

 gravitation and the law of the inverse square, Galileo's 

 Theorem, in its most general acceptation, is an immediate con- 

 sequence of the above result. 



If it should be inquired whether the hypotheses of general 

 physics, like those of physical astronomy, admit of being de- 

 rived from ulterior conditions, I should decidedly say they are 

 not so derivable. For as the philosophy of general physics 

 consists of fundamental principles perfectly intelligible from 

 sensation and experience, and of mathematical deductions 

 therefrom, no further research is either needed or possible, 

 the results being reached by means whereby alone complete 

 human knowledge of nature is attainable. These physical 

 principles must therefore be referred to causes that are " not 

 mechanical "" (to use Newton's expression), and may conse- 

 quently exist by the immediate Will of the Author of the 

 Universe. It may further be remarked that in this scheme of 

 philosophy no place is left for the exercise of the imagination ; 

 and accordingly Newton has laid down the rule (not enough 

 attended to by some modern physicists), " somnia temere con- 

 fingenda non sunt." 



It is right that in connexion with the foregoing views I 

 should advert to the many excellent treatises on physics which 

 have been published in recent times both in England and on 

 the Continent. These, one and all, are devoted to establishing 

 physical facts, and expressing the laws by which they are 



