368 



NA TURE 



[Feb. 1 8, iSS6 



subsist, because it is dynamically unstable. It does not, 

 then, seem worth while to consider the remarks made on 

 that passage. 



With regard to the first proposition that, if the moon 

 separated from the earth near the present earth's surface, 

 it can only have subsisted as a flock of meteorites, my 

 own words may be quoted as follows : — 



" The planet then separates into two masses, the larger 

 being the earth and the smaller the moon. I do not 

 attempt to define the mode of separation, or to say 

 whether the moon was initially more or less annular. At 

 any rate it must be assumed that the smaller mass 

 became more or less conglomerated, and finally fused 

 into a spheroid, perhaps in consequence of impacts 

 between its constituent meteorites, which were once parts 

 of the primaeval planet. Up to this point the history is 

 largely speculative, for, although the limiting ellipticity of 

 form of a rotating mass of fluid is known, yet the condi- 

 tions of its stability, and a fortiori of its rupture, have not 

 as yet been investigated. ... At some early stage in the 

 history of the system the moon has conglomerated into a 

 spheroidal form." ' 



When, however, Mr. Nolan goes on to his second 

 proposition, and states that this amounts to saying 

 that the moon must have been a ring of fragments 

 revolving in the plane of the equator, and that such 

 a ring must be uniformly distributed and therefore in- 

 competent to raise frictional tides, it is not easy to 

 follow him. Is there any objection to the existence of a 

 fiock of meteorites ? And would not such a flock raise 

 tides in the planet which, if subject to friction, would 

 introduce forces tending to make the meteorites recede ? 

 It seems that there is no such objection, and that the 

 flock of meteorites would follow the same fate as the 

 satellite when conglomerated in a single mass. 



The difficulties which are raised by the author in the 

 conception of the conglomeration are such as meet us in 

 all evolutionary theories, and whether or not it is possible 

 as yet to see our way mentally through the changes 

 which may have taken place, yet it is generally admitted 

 that conglomeration took place in some way. 



He then points out that no other satellite is traceable 

 up to the surface of its planet, and concludes that it is a 

 coincidence that the masses and periods of the moon and 

 earth are apparently such as fit into the theory. No one 

 has pointed out the non-existence of such a satellite more 

 clearly than 1,- but the absence of reference to my work 

 seems to show that Mr. Nolan has not looked at it. It is 

 not then surprising to read ; " Is it not very illogical to 

 suppose that the moon originated in a way which cannot 

 have been the way of origin for other planets and satel- 

 lites ? " And the reader of this sentence would hardly 

 think that my position is that there is a probability that a 

 cause which was subordinate in the history of the other 

 planets was predominant in the case of the moon and 

 earth, and that it is proved numerically that in the ter- 

 restrial sub-system the actual distribution of masses and 

 momenta (the factors governing tidal friction) differs at 

 least as much from the corresponding factors in the 

 other planetary sub- systems as the supposed modes of 

 evolution. 



On p. 13 we read : — 



" There is a law, according to which two heavenly 

 bodies cannot revolve about their centre of gravity with 

 their surfaces nearly in' contact, unless one be smaller 

 than the other by a certain amount, and, further, that the 

 small one be denser than its companion by a certain 

 value." 



I do not know where to find the proof of such a law, 

 and at the present moment am disposed to doubt its 

 correctness. 



' Phil. Trans., part 2, i83o. pp. 83o-t 

 = " On the Tidal Friction of a Plai 

 Phil. Trans., part 2. 18S1). 



attended by several Satellites " 



Next, on p. 14, we find : — 



" Rapid rotation would never cause a quantity of the 

 matter of a body to become piled up at one particular 

 place, and form into a separate single body there of any 

 appreciable size." 



Now very recently M. Poincar^ has rigorously proved in 

 a very remarkable paper' the possibility and even the 

 dynamical stability of such a "piling up," and has given 

 a sketch of the mode of separation of a portion of the 

 mass of rotating fluid. In a paper of my own, now 

 nearly finished, the same problem is treated, but from a 

 different point of view. 



It will be perceived from the quotations that the 

 pamphlet is true to its title, and refers almost entirely to 

 the genesis of the moon. This affords some proof that 

 my speculative remarks hazarded as to the mode of 

 origin of the moon, were not so guarded as was intended. 

 The justice of the third of Mr. Nolan's propositions may, 

 however, be denied, and certainly the theory cannot be 

 held to depend on the genesis of the moon at Xht present 

 surface of the earth. 



The present opportunity will be convenient for a short 

 reiteration of my point of view with regard to the whole 

 subject. 



In tidal friction we have a vera, causa of modifications 

 in the configuration of the earth and moon. If we adopt 

 provisionally the hypothesis of an adequate lapse of time, 

 we can trace the changes, and find that the obliquity of the 

 ecliptic, the eccentricity of the lunar orbit, and its inclina- 

 tion to the ecliptic (all unmentioned by Mr. Nolan), the 

 lunar periodic time, and that of the earth's rotation, are 

 co-ordinated together by supposing that the moon first 

 had a separate existence at no great distance from the 

 present surface of the earth, and with small differential 

 motion with respect thereto. Then it is maintained that 

 this co-ordination is so remarkable as to give good reason 

 for accepting the hypothesis as in accordance with truth. 

 Concerning the earlier stage in which the moon may be 

 supposed to have separated from the earth, nothing more 

 than conjecture is possible, but undoubtedly the condition 

 adduced by Mr. Nolan escaped my notice. 



In examining the rest of the solar system, it is found 

 that, amongst other things, the Martian satellites afford a 

 striking confirmation of the influence of tidal friction, and 

 that the system of the moon and earth presents features 

 so distinct from those of the other planets, as to justify 

 the belief that tidal friction, subordinate in its influence 

 on the other systems, may have been predominant in our 

 own. The theory is also found to throw light on the 

 distribution of satellites in the solar system. 



It is as yet too soon to say how far these views embody 

 the truth, but even should they be found untenable, yet 

 certainly the determination of the effects of tidal friction 

 on a system of planets and satellites is a problem of 

 physical astronomy which was well worthy of attack. 



G. H. Darwin 



ON THE SOUND-PRODUCING APPARATUS 

 OF THE CICADAS 



HAPPENING to refer to Prof. Jeffrey Bell's " Com- 

 parative Anatomy and Physiology " on the question 

 of the sounds produced by insects, I read, with reference 

 to the Cicadas :— " The sound seems to be produced by 

 the vibration of membranes, placed on either side of the 

 stigmata of the metathorax, and set in motion by the 

 respiratory air " (p. 389). 



As this wind-instrument theory of Landois seems to be 

 supplanting, in our text-books and popular natural 

 histories {e.g. Cassell's), the drum theory advocated by 

 Reaumur and the earlier writers, I think it permissible to 

 draw attention to certain observations I made on this 



'Sur I'^quilibre d'u 



