September, 1903. 



KNOWLEDGE 



197 



they were ideally riijid. Cosmic bodies, however, suns and 

 planets alike, are actually plastic spheroids ; they can, to 

 lie sure, be treated without sensible error as attractive 

 points when their distances are very great relatively to 

 their diameters ; but uj)on a closer approach inequality 

 of action supervenes. The component parts of the 

 gravitating masses respond, each individually, and in a 

 measure independently, to the graduated pulls exercised 

 upon them, and tidal strains begin variously to take 

 effect. 



Their historical significance was in part divined by Kant. 

 His penetration of so recondite a secret is truly astonishing. 

 A struggling young pedagogue in a remote Prussian 

 province, profoundly learned, though no more than half- 

 skilled in technical acquirements, saw by intuition what 

 escaped the acumen of all the great geometers of the 

 eighteenth century, namely, that the moon turns one 

 perpetual face towards the earth, because its primitive 

 rotation was stopped by the friction of earth-raised tides. 

 He perceived besides that a reciprocal action of the same 

 kind must affect the earth, and will continue to affect it 

 until the day coincides in length with the month. Nor did 

 he fail to point out that, m a molten state of the globes, 

 the process would advance with comparative rapidity. To 

 one solitary thinker, then, it became apparent, already in 

 1754,* that oceanic tides are, in cosmogony, of negligible 

 importance compared with bodily tides. 



There is no substance in natiu-e that will not change its 

 shape through prolonged stress, and the more readily the 

 nearer it approaches to the fluid condition. Tlie measur- 

 able heaping-up of the waters on the earth's surface at 

 the bidding of the moou is thus a differential effect. It 

 serves to gauge the relative mobilities of the solid globe 

 and of its liquid envelope. If the foi-mer did not yield at 

 all to the pull so readily obeyed by the latter, the tides 

 would in fact be greater than they actually are in the 

 proportion of about three to two, the ratio indicating for 

 the earth an effective rigidity at least equal to that of 

 steel, t If there were no discrepancy in rigidity between 

 the various parts of our terraqueous world there would be 

 no perceptible tides. The ocean and the bed of the ocean 

 would rise and fall together, and to the same extent. In 

 the far past, however, there was no discrepancy. The 

 viscous earth took, as a whole, the fonn momentarily 

 imjjressed upon it by the unequal attractions of the sun 

 and moon on its variously distant sections, with the 

 upshot of bringing the year, month, and day into rela- 

 tions so familiar as to appear inevitable. 



But tidal friction does not merely act as a check upon 

 rotational speed. One element of motion in a system 

 cannot be altered without some counter-change in the 

 others. They are coupled up together like a train of 

 geared wheels. From the principle of the conservation of 

 moment of momentum, we know with certainty that a loss 

 in one direction must te compensated by a gain in some 

 other. Tidal friction had then reactive c«msequences. 

 They were first adverted to by Julius Eolx'rt Mayer in 

 1848,+ and were brought prominently into view in the 

 series of investigations begun by Professor Darwin in 

 1879. The rotational momentum removed from the earth 

 by the drag of a circulating wave of deformation must 

 • assuredly have reappeared in some other part of the 

 system. It was restored, in fact, by tlie widening of the 

 lunar orbit § Concomitantly with the slackening of the 



• "Sammtliche Werke," Bd. VI., pp. 5-12, 1839. 



t Gr. H. Darwin, " Ency. Brit.," art. Tides. 



X " Dynamik dos Himmels,'' p. 411. 



§ Darwin, Phil. Traiis., Vol. CLXXII., p. 528. 



earth's axial rate, the moon retreated from its surface 

 ] lulled forward by the tidal crest continually in ailvance 

 of its position. This redressed the balance by increasing 

 orbital momentum, while at the same time diminishing 

 the moon's linear velocity. The importance of this 

 secondary frictional effect in the history of the earth- 

 moon system was the virtual discovery of Professor 

 Darwin. 



That system occupies a critical situation in the solar 

 cortege. Tlie planets interior to it have no satellites ; the 

 planets exterior to it (Neptune making probably only an. 

 apparent exception to the i-ule) have two or more. The 

 earth alone is trulj' binary ; and the moon is not only its 

 solitaiT companion, but it is by far the largest companion- 

 body, relatively to the mass of its primarv-, to be found 

 within the precincts of the solar domain. These circum- 

 stances are certainly not disconnected one from the other, 

 and they obviously depend upon a single cause. Solar tidal 

 friction was here the determining factor. The apportion- 

 ment of satellites to the various planets was, beyond doubt, 

 to a great extent prescribed by the degrees of retarding 

 power over their axial movement brought to bear through 

 the agency of sun-raised tides in their still plastic bodies. 

 Hence the disruptive rate of spinning needed for the 

 separation of satellites was never attained by either Mercury 

 or Venus ; they remained moonless for all time, and 

 exposed, through the cutting down of their rotational 

 velocity, to uncompensated extremes of temperature. How 

 the earth was to fare in both respects, long hung in the 

 lialance. Eighth' to forecast its destiny would indeed 

 have demanded no common perspicuity in an intelligent 

 onlooker. Although the solar drag upon its rotation had 

 no more than one-eleventh its power over that of Yenus, 

 it nevertheless sufficed during uncounted ages to hinder 

 acceleration from reaching the pitch involving instability. 

 Our embryonic planet had long ceased to be nebulous, and 

 had in fact shrunk by cooling nearly to its present dimen- 

 sions before the die was cast. Then, at last, the hurrying 

 effects of contraction prevailed over slowing down by tidal 

 friction, axial speed overbore equilibrium, and the spheroid 

 divided. Now globes thus far advanced in condensation 

 are apt to split less unequally than globes in a more 

 primitive stage; and the moou, Ijecause late-bom. was of 

 large size. Its mass is ^\ that of the earth ; the masses of 

 Titan and Saturn are as 1 to 4600 ; wliile Jupiter's third and 

 greatest satellite contains only n^p,, part of the matter en- 

 globed in the parent-body. Moreover, Professor Darwin has 

 made it clear that the satellites of Jupiter and Saturn revolve 

 now in orbits not widely remote from those at first pursued 

 by them ; the moon, on the contrary, having started on its 

 career almost, if not quite, from grazing contact with its 

 primary. Owing to these two exceptional circumstances^ 

 its considerable relative mass, and its close initial vicinity 

 — the moon wielded over the earth tidal influence incom- 

 parably more powerful than that exerted by any of its 

 compeers in the sun's realm. Within its bounds, accor- 

 dingly, the lunar-terrestrial system offers an unique 

 example of a pair of glolies, the mechanical relations of 

 which have been settled on their present basis by the 

 predominating agency of bodily tides. It holds forth, 

 too, the one case in which origin by fission was possible. 

 Professor Darwin's communication to the Eoyal Society 

 in 187!> occasioned a remarkable diversion of ideas. 

 Saturn's Eings were at last perceived to be illustrative 

 of only one among many feasible modes of cosmic growth. 

 It became clear that a single cut-aud-dried method would 

 not answer all the infinitely varied i>urposes of creative 

 design. Auuulation might have served its turn, but there 

 were alternatives. A fresh stand-point was virtually 

 attained, and the wide prospect commanded by it begins 



