258 



KNOWLEDGE ♦ 



[Sept. 15, 1S82. 



action. Thev would in this respect reseuil.le the diurnal 

 . hange of atmospheric pressure, shown by the barometer s 

 greater height at about ten forenoon and evening, and 

 depression at or al>out four morning and afternoon. We 

 must also remember, in explaining the annual solar tide, 

 that, Usides effects due to the sun's varying distAiice, there 

 are annual meteorological causes of variation. Thus there 

 are the monsoons and trade winds, clianging with the 

 seasons of the vear. These, however, ai-e not tides. [One 

 passage in this part of Sir W. Thomson's discourse was 

 very singularly reported in the 7'i//if.s-, thus : " The results 

 of ol>servation are utterly inexplicable on the theory of 

 gravitation,"— a statement calculated to delight the heai-ts 

 of Hampden, Xewton Crosland, Parallax, and the whole race, 

 in fact, of anti-gravitational paradoxers. It should hardly 

 Ije nec-essary to say that Sir W. Thomson made no sucli 

 statement. What he really said was, tliat outside the 

 movements of water due to the attraction of the sun and 

 moon, there are others not to be so explained. To some 

 newspaper reporters this would be iiuite the same thing, of 

 course, as saying that the observed movements are quite 

 inconsistent with the theory of gi-avitation. — Ed.] 



" But does not the weather change with the moon 1 " 

 [A manifest digression, carefully reported, however, as a 

 JMirt of the evidence relating to the tides.] Careful 

 observations with the barometer, thermometer, and anemo- 

 meter [that is, careful observations of the varying density, 

 temperature, and movement of the air] have failed to esta- 

 blish any relation whatever, and have proved, on the 

 contrary," that if there is any dependence of the weather 

 on the phases of the moon, it is only to a degree (juite im- 

 perceptible to ordinary observation. [To myself, constantly 

 asked to say whether the moon influences the weather, and 

 if so, in what way and in what degree, it is a comfort to 

 lind— however unexpectedly— a statement on this subject 

 hy Sir W. Thomson. Lunar and planetary influences, Mr. 

 K A. Bulley tells me, are not duly considered by science, 

 and he maiiifestly doubts me when I say they receive all 

 the attention they merit Sir W. Thomson's statement is 

 .lear enough on this point, and will, therefore, be good for 

 many readers of Knowledge (and especially, Mr. Bulley, 

 tor you). — Ed.] 



To r«-turn to the tides. 



The first rough view is that tlie moon attracts the waters 



f the earth to herself, heaping them up on her side of the 



irth. That would be the case if the earth and moon 



• i-re stuck on the two ends of a strong bar and put at 



■--.t in space — a state of things which does not actually 



xht 



Wliy does not the moon fall to the earth, as according 



t!,. Newtonian theory it is always doing 1 Because it 



■tion perpendicular to the direction in which it 



1 the continual fall produces merely a continual 



< f din ction. That is the dynamical theory of what 



.;- i to b<- called centrifugal force. The parts of the moon 



nearest to the earth are those which tend to fall most rapidly, 



th.- inrtsf.iitln stare those which tend to fall less rapidly.* 



I report, carofally copied in certain weekly 



or' nft'-T the manner of such reports — 



I _. 1 ., ronnrct wlint hod thuH f.ir been 



Mif- cartli! The report simply 



I ii c-iiiiBtant, or nearly eonntant, 



"f L-riivity of tlio two," wliicli, 



nt; to tlir; farllior and 



it the oiitorand nearer 



i.H<-, of roume; hut of 



■■ 'jrOB eornmonly consiHt. 



■Nil'. II in tho Time* of ecliptic 



■ fully reproduced in the EvgliBh 



organs! 



Now as the moon moves round the earth, constantly falling 

 towards her, so does the earth revolve round the moon, or, 

 more correctly, each revolves round the common centre of 

 "ravity of the earth and moon. Each preserves a constant 

 distance, or very nearly a constant distance, from the com- 

 mon centre of gravity of the two. The earth, as a ^v•l.ole, 

 experiences an attraction depending on the average distance 

 of its parts from the moon ; but its parts nearer to the 

 moon are more strongly attracted, and those farther from 

 the moon less strongly attracted, than those at this average 

 or mean distance. The result clearly is a tendency towards 

 the moon and from the moon. Thus, in a necessarily im- 

 perfect manner, is explained how it is that the waters are 

 l„ot heaped up on the side towards the moon, but) drawn 

 up towards the moon and fall away from the moon, so as 

 to tend to form an oval figure. [An explanation, however, 

 which is altogether misleading, as no one could l;now better 

 than Sir W. Thomson himself ; only doubtless ho felt that 

 the true explanation was beyond the capacity of most 

 of his audience. There is that tendency towards and from 

 the moon ; but the dynamical tendency is not at all to form 

 the oval figure shown in textbooks, with the longer axis 

 directed towards the moon, but to form an oval figure- 

 the section of an ellipsoidal water surface-with its shorter 

 axis directed moonwards. — Ed.] ., ^ j 



As to the ditterence of the tendencies of matter towards 

 the earth's surface, on account of lunar attraction. Sir W. 

 Thomson stated that, when the moon is either overhead or 

 directly under foot, tho weight of a body at the earth s 

 surface is diminished by a G,OOO,O00th part, as compared 

 with the weight when the moon is ninety degrees from 

 the vertical. * Or again, a plummet will be drawn aside by 

 a 12,000,000th part of its length when the moon is [ o 1 

 the vertical," says the Times report, complacently fol- 

 lowed in many weekly papers] ninety degrees from the 



''^ThTcarth does not yield under the influence of these 

 disturbing forces, because of her enormous rigidity regarded 

 as a whole. Mr. G. II. Darwin's results, referred to abo>e, 

 conclusively dispose of the theory t.hat the earth i.s a mere 

 crust, forty or fifty miles thick, and full of mo ten lava. 



There is a tide-really a tidal variation-depending on 

 the moons chang.^ of declination, which has, of coui^e, a 

 half-monthly period, the moon being on th« ,^1"'^t«';.*;;;';=^ 

 in each lunar month. This may be termed the fortnightly 

 or lunar declinational tide. [There is in fact, as Sir W 

 Thom.son subsequently explained, a slight heaping up of 

 the wat<T round the equator and lowering at the poles, 

 once a fortnight, when the moon is crossing the equator, 

 alternating with lowering round the equator, and heaping 

 up at the poles when she attains her extreme north and 

 south declination.] There is also a tidal variation which 

 I if we want to confuse an audience] we may call a tide, 

 due to th..- moon's varying distance in each lunar month. 

 This tide, whose period is, of course monthly, may be 

 called the elliptic tide, because it depends on the ellipticity 

 of the earth's orbit. *„,.,„,. 



In a n.-wspaper report it is bound, of course to appear 

 ^ the ecliptic };*, while in an ama/.ing "leader wli.c-h 

 adorned the pages of the Times for Monday, August JH, 

 this tide appears as the electric tide, "an error little to be 

 wond..re.l at," writes Sir W. Thomson in the Jimes for 

 Wednesday, August .'50,— a remark whose biting sarcasm 

 seems to have escaped the attention of the editor (or who- 



• Here, again, tlio Timcii report was utterly absurd, though hero 

 n(;ain it W08 followed ))y several scientific papers. It stated that 

 tho weight is diminished when tho moon is overhead, and increased 

 in tlio some degree when tho moon is unilcr foot, in each case by 

 onc-6,000,000th part— an egregiously incoirect statement. 



1 



