208 



KNOWLEDGE. 



[September 1, 1896. 



heap them up on the opposite side seems, on the contrary, 

 palpably absurd. Theerrorof this class of objectors .... 

 consists in disregarding the attraction of the disturbing 

 body on the mass of the earth, and looking on it as wholly 

 effective on the superficial water. Were the earth indeed 

 absolutely fixed, held in its place by an external force, and 

 the water left free to move, no doubt the ell'ect of the 

 disturbing power would be to produce a single accumula- 

 tion vertically under the disturbing body, lint it is not 

 by its whole attraction, but by the dill'crenco of its 

 attractions on the superficial waters at both sides, and on 

 the central mass, that the waters are raised." 



He then goes on to say that it has been found by 

 calculation that the moon's maximum power to disturb the 

 waters on the earth is about one 12, 560, 000th of gravity, 

 which would be suflicient to make a difference of about 

 fifty-eight inches between high and low water ; the power 

 of the sun being about two and a half times less, or about 

 twenty-three inches. 



Your correspondent, Mr. J. Creagh, seems to be under 

 the impression that if two forces are acting in opposite 

 directions on a body, that body would act as though it 

 were affected solely by the more powerful of the two forces, 

 instead of acting as though it were affected by a force 

 equal to the excess of one of the forces over the other. 

 Otherwise he would scarcely say that "the authors" (to 

 whom he frequently refers, but never by name) " fail to see 

 that the tidal power of the moon, being almost infinitesimal 

 compared to the earth's gravity force, could raise towards 

 herself in direct opposition to gravity neither a particle of 

 water, nor a grain of sand, nor any portion of matter, 

 small or large." 



This is with reference to the " suction theory." That 

 which he calls the slip theory " he dismisses on the 

 same grounds. 



In his " weight theory " I do not perceive how, by 

 " descending to the application of the idea," he arrives at 

 the conclusion that " in order to account for a rise of only 

 three feet in their height, a depth of water of between 

 three thousand and four thousand miles would be required." 



G. H. Hill. 



To the Editors of Knowledge. 



Sirs, — Will you kindly allow me to add a few remarks 

 to what I have already said upon the interesting problem 

 of the tides ? 



A satisfactory solution might be arrived at, or at any rate 

 greatly facilitated, if philosophical writers were to definitely 

 state from what source the tides derive their dynamical 

 energy. From the test-books it is difficult to say whether 

 they derive their energy from the moon, or the moon 

 and the sun combined, or from the earth's rotation ; such 

 is the ambiguity of the language used and the obscurity of 

 the ideas entertained by different authors. I may be 

 thought pedantic for not accepting without demur the 

 dictum of men much more learned than myself, and for 

 having an opinion of my own ; but I firmly believe that the 

 tides cannot derive any dynamical energy from an inert body 

 like our satellite, neither can I believe that two such dis- 

 similar bodies as the sun and moon have a similar physical 

 and mechanical effect upon the ocean. No doubt the sun — 

 unlike the moon — does have a dynamical effect upon the 

 ocean by means of its heat, but this manifests itself in the 

 form of currents flowing from the tropical to the polar 

 seas, and are movements totally distinct from and difl'erent 

 to the tides. 



Although the moon is a source of no dynamical energy 

 to the earth, the latter body may be a source of energy to 

 the moon, and by its diurnal rotation may also cause the 



tides, the centrifugal force acting more powerfully upon 

 the liquid and mobile ocean than upon the earth's solid 

 crust. 



" We cannot account," as ]\Ir. Proctor says, " for the 

 moon's peculiarity of rotation, without regarding it as due 

 to the earth's controlling inlluence " ; and this belief is con- 

 firmed by the singular coincidence that the energy of the 

 tides and the velocity of the moon's motion vary concur- 

 rently and in a precisely similar manner, proving that the 

 tides and the moon's motion in her orbit must have a 

 common cause. A\'hat else can this cause be than the 

 earth's centrifugal force ? 



In conclusion I would draw especial attention to the 

 remarkable correlation that exists between the height of 

 the tides and the lunar variation. This synchronism — that 

 the highest tides (the spring tides) should occur when our 

 satellite is in syzygy, and moving at her greatest speed, 

 and that the lowest tides (the neap tides) should occur 

 when the moon is in quadrature, and moving at her slowest 

 speed — is most significant, and must have an important 

 bearing upon the causation of the tides, although it is 

 overlooked or ignored by every author that I have come 

 across who deals with the theory of the tides. 



H. A. CoOKSON. 



A LUNAR RAINBOW. 

 To the Editors of Knowledge. 



Sirs, — On the night of June 27th last — a cloudy, dark 

 night — I was sitting in the verandah of my quarters, when, 

 just as rain, which had been imminent for some time, 

 began falling, I was astonished by seeing a distinct band, 

 which emanated from a patch of light cloud and fell in the 

 shape of a rainbow into a tank of water in front of my 

 house. The moon was shining brightly at the time, and 

 on watching the band I distinctly noticed that it had a 

 dark reddish appearance on the outside edge and was of a 

 bluish-green tinge elsewhere. There was also a reflection 

 appearing of a uniform whitish hue. 



I first noticed the lunar rainbow, as I believe it to be, 

 at 9.48 P.M., and watched it carefully till its disappearance 

 at 10.1 P.M. 



The outer arc or reflection disappeared almost im- 

 mediately, and the rainbow, as it receded, appeared for a 

 short space as of a uniform bluish -green hue, and finally, 

 before its disappearance, seemed to me of a whitish hue 

 like the outer arc at the commencement. 



R. J. D. Sair, Lieut., 



Bhamo, Upper Burma. 80th (5th Burma Bn.) M.I. 



SOME CURIOUS FACTS IN PLANT 

 DISTRIBUTION.-IV. 



By W. BoTTiNG Hemsley, F.R.S. 



WITH the exception of a brief account of the 

 new vegetation of Krakatoa, I have so far 

 only given some particulars of the plants 

 inhabiting a few of the remote islands of the 

 temperate and frigid zones of the southern 

 hemisphere. The countless islands of the Pacific Ocean 

 within the tropics, as well as the comparatively few 

 remote isolated islands in the Atlantic and Indian Oceans, 

 are no less interesting from a botanical standpoint — to 

 say nothing of their animal inhabitants. The Seychelles 

 group in the Indian Ocean is specially so. These islands, 

 upwards of thirty in number, are situated about six 

 hundred miles to the north-east of Madagascar ; the 

 largest being seventeen miles long and five in average 

 breadth, with an altitude of nearly three thourand feet. 

 Formerly most of the islands were covered with forest, the 



