246 SCIENCE PROGRESS 



Let that pass for the present. But now reckon the earth's 

 mass, at opposite points in the orbit, when it is moving 

 parallel to a component of the sun's way at the known rate 

 io~ 4 c. On one side of the orbit the denominator of the 

 above fraction would be -/('0202), and on the other side 

 >/( , oigS) ; so that the difference due to the extreme variation 

 in the earth's apparent mass in the course of six months would 

 be 7 per cent, of the real mass, or jhfth. of the average effective 

 mass, of the earth. Such a difference in the centripetal 

 acceleration would have conspicuous effects, on the reasonable 

 hypothesis, argued below, that the extra or spurious mass is 

 not subject to gravity. For the sun's perturbing force on 

 the moon is just about one-hundredth of the earth's main 

 attraction J ; and as a result Newton deduced an apsidal 

 progression at the rate of i|°, if not 3 , per revolution 

 (Principia, Book I. section ix.). 



Hence we can say that the solar system is not advancing 

 through the ether at anything near the velocity of light. It 

 is something if we can definitely make such an assertion as 

 that on physical grounds. It shows that motion through the 

 ether is at least not meaningless. 



Again, when some spurious or extra inertia is actually 

 being observed experimentally, in electrons or a particles 

 flying in a vacuum, the aspect of the vacuum tube should 

 make some difference, if the solar system were travelling at 

 an enormous speed. Probably most people would agree that 

 it is wholly unlikely that the visible cosmos is careering through 

 space at any immense velocity : but why can we say it is 



sec 6. A good approximation, when 6 is moderate, is 1 + - 6* + — Z- B*. Another 



*._„ A A 



still closer one is — ~ . See Alfred Lodge's Dift. Cal. p. 184. 



Historical Note. Heaviside's expression for the coefficient of ^v 7 in the value 

 of kinetic energy for a charged sphere was first given in Phil. Mag. April 1889. 

 (See Heaviside's Electrical Papers, Vol. II., p. 514, or my book on Electrons, 

 p. 225.) 



J. J. Thomson's expression for the coefficient of v in the value of momentum 

 is contained in his Recent Researches in Electricity and Magnetism, p. 21, published 

 in 1893. 



1 The sun's pull on the moon is greater than the earth's pull, the ratio being 

 SjE : (R/f) 2 , but the perturbing effect of the sun's pull is much less ; for the ratio 

 of the sun's perturbing force to the earth's controlling force is 



iS\E : {R\rf = 6 x 107400 3 = 1/100. 



