the Luminiferous dEther. 399 



it would be if the orbit were circular, having for its radius 

 the perihelion distance. According to Professor Morrison, 

 the perihelion distance of the great comet (6), 1882*, was 

 716,200 miles, its aphelion distance will be 5,000,000,000 

 miles, the diameter of its nucleus shortly before disappearing 

 on the solar disc was 7600 miles, the velocity at perihelion 

 295 miles per second and at aphelion 75 feet per second. 

 But little is known in regard to the density of comets ; but 

 to be on the safe side we will assume it as j~> that of water. 

 This data will reduce (18) to 13 x 10" 18 for the fraction of 

 energy lost during one of its revolutions about the sun ; and 

 as it would make a revolution in, say, 20 hours, it would lose 

 in one of our years about 57 x 10~ 16 of its energy, at ivhich 

 rate it would go on for 170 trillions of years. Similarly, at 

 its aphelion its rate of loss would be less than | x 10"" 15 of its 

 energy in more than 2000 years — the time of one revolution 

 in its orbit. 



The most careful observations and calculations have failed 

 to detect any effect due to the resistance of matter in space ; 

 and the above analysis shows that, within historic times, it has 

 in any case scarcely amounted to an infinitesimal, certainly 

 not sufficient to be measured. And when we consider that 

 our assumptions have been very largely on the unfavourable 

 side, and, further, that the energy imparted to the aether may, 

 partly at least, be restored to the body, we assume that its 

 resistance never can be measured. Laplace, when he found 

 that the force of gravitation, if propagated by an elastic 

 medium, must have a velocity exceeding 100 million times 

 that of light, concluded that astronomers might continue 

 to consider its action as instantaneous (Mecanique Celeste, 

 B. x. ch. viii. p. 22, 9035) ; so may we, with as much con- 

 fidence, continue to consider the resistance of the aether as nil. 



Equation (6) gives 



6-6(186300x5280)* 

 CT "2x 32-2 x 1-4 x 772 ~ yJ X iU ' " # (20) 



from which the specific heat of the aether may be found if its 

 temperature were known. M. Fourier, the first to assign a 

 value to the temperature of space, assumed it to be somewhat 

 inferior to the temperature at the poles of the earth, or about 

 50° C. to 60° C. below zerof. M. Pouillet, considering the 



* ' Monthly Notices of the Eoyal Astronomical Society/ vol. xliv. 2, 

 p. 54. 

 t Ann. der Ckemie, tome xvii. p. 155. 



