90 THE POPULAR SCIENCE MONTHLY. 



adopt, we are equally driven to the conception of the existence of some 

 form of matter in the celestial spaces. The fact that light and heat 

 are propagated from one part of si:)ace to another in time demands 

 that the medium of communication should possess inertia — an essen- 

 tial property of matter. According to the wave theory^ the celestial 

 bodies move in an attenuated and subtile ethereal medium ; according 

 to the corpuscular theory, they move in a perpetual shower of cor- 

 puscles emitted by the sun and stars. In both cases matter exists — in- 

 ertia exists — therefore resistance must be encountered. The smalhiess 

 of the resistance, however small we choose to suppose it, does not al- 

 low us to escape this certainty. There is resistance, and therefore the 

 movements of satellites cannot escape its influence. Nevertheless, such 

 attenuated and htdky masses as comets are best adapted to test the ex- 

 istence of a resisting medium. 



3. In the last place, it is possible that Mars may have originally 

 rotated on his axis in five or six hours, but that the tidal rotation- 

 retardation produced by the action of his moons might have brought 

 about its present rotation-period. It is evident that the solar tides, on 

 a planet so small and so remote from the sun, must be inappreciable ; 

 and, at first sight, the lunar tides produced by such small masses might 

 be supposed to be equally insignificant. But it must be recollected 

 that the tide-generating power of a moon is (other things being equal) 

 inversely proportional to the cube of its distance ; so that nearness 

 might more than compensate for smallness of mass. To be more spe- 

 cific : In the mathematical language, the tide-generating power is in 

 proportion to the Thus, 



Diameter of Primary X Mass of Satellite 

 (Distance of Satellite.)' 

 for example, let us suppose the diameter of our moon to be twenty 

 times the diameter of the inner satellite of Mars, and both moons to be 

 equally dense ; then the mass of our moon would be 8,000 times that 

 of the Martial satellite. Taking the diameter of the earth as equal to 

 twice the diameter of Mars (and it is not so great), and the distance of 

 our moon from the center of the earth to be forty-one and a half times 

 the distance of the inner satellite from the center of Mars, we then 

 have the tide-generating power of our moon acting on the earth, will 



be to that of the inner satellite acting on Mars as — to 1, or as 



r^^A^c) to 1, or as -—.J— to 1, or as 1 to ^\. Hence, the tide-generating 



power of this small satellite would, in consequence of its nearness to 

 Mars, be about four and a half times as great as the tide-generating 

 power of our moon on the earth. 



This view, however, is not free from the most serious p)hysical 

 difficulties. For it is evident that the tidal rotation -retardation pro- 

 duced by the moons would be limited by the final condition, that the 



