316 THE POPULAR SCIENCE MONTHLY. 



pressible, Motionless fluid, originally at rest, but set in motion and 

 kept in motion by solids, moving to and fro, or whirling round in any 

 manner, through a finite space of it," would explain the attractions 

 just described. Moreover, he is persuaded by other effects besides 

 those of light, that, in the interplanetary spaces and in the best arti- 

 ficial vacuum, the medium which remains has " perfectly decided 

 mechanical qualities, and, among others, that of being able to trans- 

 mit mechanical energy, in enormous quantities : " and he cherishes 

 the hope that his mathematical theorems on abstract hydrokinetics 

 are of some interest in physics as illustrating the great question of the 

 eighteenth and nineteenth centuries : Is action at a distance a reality, 

 or is gravitation to be explained, as we now believe magnetic and 

 electric forces must be, by action of intervening matter ? 



In 1869 and 1873, Prof. Challis, of Cambridge, England, published 

 two works on the " Principles of Mathematical Physics." They em- 

 body the mature reflections of a mathematical physicist at the ad- 

 vanced age of threescore years and ten. Challis believes that there 

 is sufficient evidence for the existence of ether and atoms as physical 

 realities. He then proceeds to say: "The fundamental and only 

 admissible idea of force is that of pressure, exerted either actively by 

 the ether against the surface of the atoms, or as reaction of the atoms 

 on the ether by resistance to that pressure. The principle of deriving 

 fundamental physical conceptions from the indications of the senses 

 does not admit of regarding gravity, or any other force varying with 

 distance, as an essential quality of matter, because, according to that 

 principle, we must, in seeking for the simplest idea of physical force, 

 have regard to the sense of touch. Now, by this sense, we obtain a 

 perception of force as pressure, distinct and unique, and not involving 

 the variable element of distance, which enters into the perception of 

 force as derived from the sense of sight alone. Thus, on the ground 

 of simplicity as well as of distinct perceptibility, the fundamental idea 

 of force is pressure." As all other matter is passive except when 

 acted upon by the ether, the ether itself, in its quiescent state, must 

 have uniform density. It must be coextensive with the vast regions 

 in which material force is displayed. Challis had prepared himself 

 for the elucidation and defense of his dynamical theory by a profound 

 study of the laws of motion in elastic fluids. From the mathematical 

 forms in which he has expressed these laws he has attempted to de- 

 rive the principal experimental results in light, heat, gravitation, elec- 

 tricity, and magnetism. Some may think that Mr. Challis has done 

 nothing but clothe his theory in the cast-off garments of an obsolete ' 

 philosophy. If its dress is old, it walks upon new legs. The inter- 

 play between ether and atoms is now brought on to the stage, not as a 

 speculation supported by metaphysical and theological arguments, but 

 as a physical reality with mathematical supports. I should do great 

 injustice to this author if I left the impression that he himself claimed 



