THE LAST RIDDLE 453 



This is part of the puzzle. Another is the apparently in- 

 stantaneous propagation of gravitation. Attraction is a force, 

 and every other force with which we are acquainted requires a 

 finite time for its extension through space. Endless endeavours 

 have been made to discover a rate of propagation. Laplace was 

 among the many who tried it. He thought for a time that 

 he had; latterly he gave it up. Very recently a German 

 mathematician, Gerber, thought to deduce the rate from the 

 motion of perihelion of Mercury. 1 This is estimated at forty- one 

 seconds in a century, and according to Gerber's computations 

 this would indicate a rate of propagation for gravitation similar 

 to that of light and electric waves. It remains as yet merely 

 a plausible theory. 



A further mystery of gravitation is that it cannot be screened. 

 It acts apparently with the same force through a planet as through 

 empty space. When the planetary bodies are in conjunction 

 that is to say, in a straight line from the sun one with the other 

 they swerve slightly perforce of their own mutual attractions; 

 but not otherwise. It needs be said that in this regard gravita- 

 tion does not stand absolutely alone. The same thing is true 

 of magnetism. We are equally unable to devise a magnetic 

 screen. If we could do either we should have perpetual motion, 

 and, more than this, we should be able to navigate space. 



It is obvious that so long as we know nothing of the theory 

 or "cause" of the most familiar fact of our daily lives, we 

 cannot advance very far. Countless efforts have been made to 

 break through this impasse ; none have succeeded that is, 

 if we bar the philosophers. Readers with a taste for such 

 things will doubtless find interest in a recent volume of Pro- 

 fessor Wilhelm Ostwald, 2 wherein the difficulties which have 

 beset a considerable line of hard-headed thinkers, from Newton 

 to Kelvin, are lightly brushed away. 



In a scientific sense, probably the most noteworthy attempt 

 towards an explanation was the hypothesis proposed by Lesage, 

 a modest physicist of Geneva, about a hundred and fifty years 

 ago. Lesage, who is in no wise to be confounded with the 

 author of Gil Bias, suggested that the facts ofg gravitation 

 might be accounted for by conceiving of an infinite rain or 

 hail of infinitesimal particles, driving about in every direction 



1 Zeitschr. f. Math. u. Phys. 1898, II. 

 2 Vorlesungen iiber N aturphilosophie ; Leipzig, 1902, p. 193. 



