March, 1913. 



KNOWLEDGE. 



93 



resisting medium, since owing to its minute size a 

 proportion of its momentum would be more easily lost, 

 and it might thus have been brought to its present 

 position close to the planet by revolving against 

 resistance. This suggestion gains weight from the 

 fact that there are known to be over six hundred 

 similar bodies to the two Martian satellites, revolving 

 between that planet and Jupiter, and it seems quite 

 likely that two of these asteroids may at some time 

 have come within the sphere of the planet's 

 influence and so have attained their present 

 positions. 



Apropos of these satellites, it is interesting to dwell 

 for a moment on the remarkable verification of 

 prophecy brought to light by their discovery. At the 

 time when " Gulliver's Travels " was written a good 

 deal of ridicule had been cast upon contemporary 

 astronomical research, and for purposes of satire 

 Dean Swift caused Gulliver to relate how, in the 

 island of Laputa, astronomers had discovered two 

 satellites to Mars, one of which revolved around him 

 in ten and the other in twenty-one and a half hours. 

 Forty years ago such a phenomenon would 

 have been considered quite impossible and only a 

 flight of the wildest imagination, and when it is 

 considered that the laws of chance would be almost 

 infinity to one against the fulfilment of the prophecy 

 it renders its realisation all the more remarkable. 



We will now briefly recapitulate the arguments for 

 both hypotheses of lunar origin. The whole subject 

 turns on the question of a resisting medium. If this 

 still exists it would invalidate the calculations of the 

 Moon's motion previously described, and would 

 render the probability of its having been captured 



almost certain. This theory, indeed, appears to 

 have no very great drawbacks attached to its 

 acceptance, and, according to the supposed origin 

 of the solar system, to be most concordant with it. 

 Moreover, there is an outstanding inequality in the 

 movement of the Moon which up to the present has 

 defied the utmost efforts of astronomers to assign to 

 anv known cause, but which might be accounted for 

 by the effects of orbital motion against resistance. 

 Supporters of the fission theory, however, have to 

 contend with many very real difficulties, amongst 

 them to account for the Earth ever having had the 

 necessary rotational velocity, and even if this is 

 conceded, the difficulty of understanding how lunar 

 disruption could be prevented until the Moon 

 attained to a sufficient distance to revolve in 

 safety. On the other hand there is the wonder- 

 ful evidence in its favour just detailed and 

 which, if it is a coincidence, must be regarded 

 as one of the most extraordinary in the annals 

 of astronomy. It must also be remembered that 

 the ability of tidal friction to retard rotation is 

 an established fact, as shown by the Moon's own 

 rotation, the only doubtful factor being the time 

 required to produce any effect. 



Our satellite would gain an additional interest if 

 the capture theory is ever proved to be correct, since 

 there seems to be a greater fascination in scrutinising 

 a world having an entirely different origin from our 

 own, than in merely looking upon a fragment of our 

 earth. Let us hope that some day the mystery may 

 be cleared up, and that we may know more concern- 

 ing the past history of our companion and nearest 

 neighbour in the cosmos. 



CORRESPONDENCE. 



AN "IDEAL" MUSEUM AND ITS GUIDE. 

 To the Editors of " Knowledge." 



Sirs, — The " Provincial Curator," in his contribution to 

 " Knowledge " of January, does an unintentional injustice to 

 the Trustees and Curator of the London Museum. The 

 " Guide to Kensington Palace " is, apparently, a reprint of a 

 former edition in which have been included some notes on the 

 London Museum by the author, but these notes have no 

 authority from, and were never submitted to, or revised bv, 

 those responsible for the London Museum and its arrange- 

 ment. Your contributor has, perhaps, been misled by this 

 " guide " into thinking that the series of objects are not 

 chronologically arranged, but this is a mistake so far as the 

 rather limited arrangement of the cases in their present 

 temporary home will permit. 



If your contributor would call again at the London Museum 

 and ask for the Curator, or one of his assistants, he would 

 have much pleasure in explaining to him the motive and 

 system of arrangement and in receiving from him any 

 suggestions for their improvement. 



GUY FRANCIS LAKING, M.V.O., F.S.A., 

 Keeper and Secretary. 

 The London Museum, 



Kensington Palace. 



[Our Contributor " A Provincial Curator " did say (see 

 " Knowledge" for January, page 16) that the Keeper and 



Secretary of the London Museum did not write the Guide, 

 and now we learn that it was never even submitted to him. 

 On the face of it, this behaviour seems to be foolish and dis- 

 courteous as well as detrimental to the best interests both of 

 the museum and of the author of the Guide, who, we think, 

 should come forward and give some explanation. — Eds.J 



THE FOURTH DIMENSION. 



To the Editors of " Knowledge." 



Sirs, — In reply to Mr. H. Stanley Redgrove's letter in the 

 February issue of " Knowledge," I may say that it is not 

 necessary for me to see his book, nor for him to state in 

 detail his argument for the real existence of the fourth 

 and other dimensions. What he has already stated is quite 

 enough, namely, that his argument is that the existence of one 

 dimension implies that of a second, a second that of a third, 

 a third that of a fourth, and so on. I have pointed out that 

 experience — direct perception of some kind — is the only 

 possible basis of our knowledge of what exists, and that this 

 experience gives us the law that where there is one dimension 

 there must be three, but that it gives us absolutely nothing 

 else — nothing whatever to support an argument that the 

 existence of one dimension implies that of a second, and so 



on ' ..... JOHN JOHNSTON. 



Hendon, N.W. j j 



