236 



NATURE 



\yan. 3, 1889 



sun on the materials of the comet, and that of the comet on the 

 nebulous atmosphere by which it was surrounded. 



Olbers was driven to consider the repulsive action of the comet 

 on its atmosphere in order to explain the many luminous sectors 

 visible in the comet in question. To this he also ascribed 

 the gradual rise of successive envelopes, so well illustrated 

 subsequently by the comet of Donati. 



The energy of electrical repulsion depends upon the amount 

 of surface of the. bodies concerned, whereas the attraction of 

 gravity depends upon the masses of the bodies. Small things 

 have more surface in proportion to their masses than large ones, 

 and there will therefore be attraction or repulsion between the 

 sun and the particles composing comets according as the differen- 

 tial effect of the two opposite forces is repulsive or attractive. 

 In the very small particles, the electrical repulsion will be stronger 

 than the attraction due to gravitation, while in the larger 

 particles the two forces may balance each other, or gravitation 

 may preponderate. Only the finest particles composing the head 

 of a comet are therefore repelled to form the tails. 



BesseP considerably modified this hypothesis. He considered 

 that the action of the sun on the comet represented a polar 

 force. 



M. Faye has more recently held that this repulsive action is 

 due to the radiant energy of the sun, and that it has an intensity 

 inversely as the square of the distance, and proportional to the 

 surface and not to the mass of the moving particles. Its action 

 would therefore be in the inverse ratio of the density of the par- 

 ticles upon which it acted ; it would vary with every difference 

 of cometary constitution ; it would be inappreciable on the 

 nucleus itself ; (the idea being, of coarse, that the nucleus wa^ 

 a solid body) ; and it would be most effective in the case of the 

 rarest vapours. The important part of M. Roche's later memoir 

 consists in testing these views of repulsion, to determine whether 

 the forms of comets could be explained by its introduction. 



One result is very striking : the tail towards the sun demanded 

 by gravitation alone at once disappears. The limiting surfaces 

 which Roche's calculations demand are so very like some of 

 the surfaces actually observed in the head of a comet, where they 

 can be best seen, that it is suggested that the movement of the 

 particles takes place in the precise direction where they would 

 How according to M. Roche's mathematical investigations. 



Hence we are justified in attributing some cometary pheno- 

 mena to the flow of matter aciing under the influence of attrac- 

 tion and solar repulsion.- In concluding his memoir Roche 

 points out (p. 393) that the hypothesis of a repulsive force 

 acting along a radius vector, and varying inversely as the square 

 of the distance, and only acting on matter reduced to a state 

 of great rarefaction, gives figures identical with those observed. 

 We see the germ of the tail is the part of the atmosphere 

 the furthest removed from the sun, and it is easy to explain the 

 enormous development of the emission of cometary particles 

 near perihelion. The existence of a repulsive force which 

 cuunterbalances the solar attraction M. Roche therefore considers 

 established by his researches. 



It must, however, be at once stated that much remains to be 

 done before all the help that M. Roche's work can afford can be 



distance from perihelion. But there may be another reason. 

 If the outflow along the limiting surface is an outflow of solid 

 particles, the solar repulsion will not be effective until 

 collisions have reduced this dust to vapour. We shall still 

 therefore have the quasi-conical surface turned towards the sun,^ 

 though it will be soon destroyed. Many of the phenomena pre- 

 sented by jets and excentric envelopes may be thus caused, and 

 the very complicated phenomena presented by Coggia's comet, 

 and others in which the section of the cone presents the appear- 

 ance of birds with their wings more or less extended, do not 

 seem opposed to this view. 



J. Norman Lockyer. 



( To be continued. ) 



Roche's theoretical construction of the head of a comet, a 

 repulsive force being taken into account. 



utilized, and there is little question that the outflow in the solar 

 direction has not been so entirely abolished as his figures 

 indicate. This, however, may to a certain extent depend upon 

 the fact that the observations of comets have been made at some 



' Besfel's paper " On the Physical Constitution of Halley's Comet " is 

 print d in the Connaissance des Temps, 1840. 

 ^ See Annates de fObsematcire de Paris, vol. v. 



PRELIMINARY NOTE ON KEELING ATOLL, 

 KNOWN ALSO AS THE COCOS ISLANDS. 



A/TR. JOHN MURRAY, of the Challenger Expedition 

 ■'■-'- Office, has forwarded to us the following letter, which 

 he has received from Dr. Guppy :— 

 Dear Mr. Murray, — 

 During my sojourn of nearly ten weeks in these islands, I 

 was able to make a fairly complete examination of them. 

 Here, I can only refer to some of the new features of this atoll 

 which my investigations have disclosed, and must leave the 

 details to be subsequently worked into a general description of 

 the islands. Regarding myself as very fortunate in being able 

 to examine the only atoll visited by Mr. Darwin — the atoll, in 

 fact, which gave rise to the theory of subsidence — I at once set 

 about making observations, without reference to any particular 

 view of the origin of coral-reefs. I examined all the islands and 

 islets, more than twenty in number, making a separate descrip- 

 tion of each, and reaped the benefit of the fact that this atoll 

 has been occupied for more than half a century by residents 

 interested in their surroundings The result has been to con- 

 vince me that several important characters of these islands 

 escaped the attention of Mr. Darwin, partly owing to his 

 limited stay, partly also due to his necessarily defective infor- 

 mation of the past changes in the atoll. The features, in fact, 

 that escaped his notice, throw considerable light on the mode of 

 origin of these lagoon islands, and give no support to the theory 

 of subsidence. 



In the first place, I have ascertained that Keeling Atoll consists 

 essentially of a ring of horse-shoe or crescentic islands inclosing 

 a lagoon and presenting their convexities seaward. The cres- 

 centic form is possessed in varying degrees by different islands : 

 some of the smaller ones are perfect horseshoe atollons, and 

 inclose a shallow lagoonlet ; others, again, exhibit only a semi- 

 crescentic form ; whilst the larger islands have been produced by 

 the union of several islands of this shape. The whole land- 

 surface, however, is subject to continual change. The extremi- 

 ties of islands are often being gradually swept away or extended. 

 Some islands are breached during heavy ga'es, others are joined, 

 so that by the repetition Of these changes the island in the 

 course of time loses its original form. Hence it is that, although 

 the crescent is the primitive shape of each island this structure 

 is partly disguised in the case of some of the larger islands by 

 the union of several of smaller size. The Admiralty chart 

 gives but an imperfect idea of the true shape of the islands ; 

 but, notwithstanding, its inspection will prove very ins'ructive. 



In truth, Keeling Atoll exhibits in an incomplete manner the 

 features of the large compound atoll of the Maldive Group. If 

 it was considerably larger and possessed a less protected lagoon, 

 so that open-sea conditions prevailed in its interior, it would 

 have all the features of a compound Maldive atoll— that is, an 

 atoll consisting of a circle of small atolls or atollons. In its 

 original condition, however, it was an atoll consisting of a 

 circle of crescentic islands. Such it is essentially now, but 

 extensive changes have often partly disguised this feature. 



Before proceeding to explain the origin of the incompleted 

 compound atoll of the Keeling Islands, it will be necessary 

 to dwell on the exaggerated prevailing notion of an atoll. 

 This kind of coral-reef is usually described as a circular reef 

 inclosing a deep basin or lagoon ; but this description only 

 applies to very small atolls less than a mile across. By drawing 

 a section on a true scale of an atoll of average size, like Keeling 

 Atoll, it will at once become apparent that such a description 

 ' Although this does not figure in Roche's diagrams, Faye gives it in his 

 lectures on the '" Forms of Comets." 



