NATURE 



\ytme 20, 1872 



aqueous vapour in condensing developes positive elec- 

 tricity. No unusual development of electricity has ever 

 been detected by him in a cloud when no rain is falling. 



The above results, though falling short of what has to 

 be done to complete the theory, are yet definite, and hence 

 valuable, the more so if supported by other observers 

 placed in equally favourable situations. But of the varia- 

 tions /// intensity of positive or negative electricity nothing 

 has been said. 



Besides the fixed instruments at the Observatory others 

 are used on the mountain. Gases are collected from 

 cracks in the earth's crust, tubes being let down into 

 them, and the gas sucked up by a kind of bellows to be 

 examined at leisure. A portable spectroscope is also used 

 during eruptions, and there is a larger one by Hoffman in 

 the Observatory. From this Observatory we have re- 

 ceived valuable information, and it is much to be regretted 

 that equally efficient observatories have not been estab- 

 lished in different parts of the world. Many portable and 

 cheap instruments have been invented, most of which are 

 described by Mr. Mallet in the "Admiralty Manual of 

 Scientific Inquiry;" but there ought to be three or four 

 as delicate as that on Mount Vesuvius. It is a pity that 

 no observatory has ever replaced the ancient one of 

 Empedocles near the summit of Etna, or even at Nicolosi, 

 where the valuable servicer of Dr. Gemellaro might have 

 been obtained. This would have been the more interest- 

 ing, as Palmieri can detect shocks caused by that volcano, 

 though the distance is enormous. With a third observa- 

 tory, say in the Philippine Islands, we could not fail to in- 

 crease our knowledge enormously. 



From long practice Palmieri is able to predict erup- 

 tions. We remember well when we were enjoying his 

 hospitality at the beginning of last year how he said, 

 "This is a small eruption, but there is going to be a great 

 one ; I do not say it will be soon, it may be a year, but it 

 will come." In almost e.\actly a year the great eruption 

 did come. George Forbes 



ON THE DISINTEGRATION OF COMETS 



''PHE main design of the following paper is to present 

 J- at one view the historical evidence of the gradual 

 disintegration of periodic comets. A few preliminary 

 remarks, however, in regard to the received theory of 

 comets and meteors, may not be destitute of interest. 



The fact that in several instances meteoric streams 

 move in orbits identical with those of certain comets was 

 first fully established by the researches of Signor Schiapa- 

 relli. The theory, however, of an intimate relationship 

 between comets and meteors was proposed and advocated 

 by the writer several years previous to the publication of 

 Schiaparelli's memoiis. In an article written in July 

 1861, and published in the " Danville Quarterly Review" 

 for December of that year, it was maintained — 



1. That meteors and meteoric rings "are the rt'i.'fovj of 

 ancient, but now disintegrated comets whose matter has 

 become distributed around their orbits."* 



2. That the separation of Biela's comet as it approached 

 the sun in December 1845 was but one in a series of 

 similar processes which would probably continue until the 

 individual fragments would become invisible. 



3. That certain luminous meteors have entered the solar 

 system from the interstellar spaces. t 



4. That the orbits of some meteors and periodic comets 

 have been transformed into ellipses by planetary perturba- 

 tion. And — 



5. That numerous facts — some observed in ancient and 

 some in modern times — have been decidedly indicative of 

 cometary disintegration. 



What was thus proposed as theory has been since con- 

 firmed as undoubted facts. When the hypothesis was 



* The name oi comeioids was accordingly proposed for luminous meteors. 

 + Others, it was supposed, might have originated within the system — a 

 view which the writer has not wholly ab.indoned. 



originally advanced, the data required for its mathematical 

 demonstration were entirely wanting. The evidence, 

 however, by which it was sustained was sufficient to give 

 it a high degree of probability. 



The existence of a divellent force by which comets 

 near their perihelia have been separated into parts, is 

 clearly shown by the facts enumerated in the following 

 lines. Whether this force, as suggested by .Schiaparelli, 

 is simply the unequal attraction of the sun on diflcrent 

 parts of the nebulous mass, or whether, in accordance 

 with the views of other astronomers, it is to be regarded 

 as a cosmical force of repulsion, is a question left for 

 future discussion. 



1. Seneca informs us that Ephoras, a Greek writer of 

 the fourth century B.C., had recorded the singular fact of 

 a comet's separation into two distinct parts.* This state- 

 ment was deemed incredible by the Roman philosopher, 

 inasmuch as the occurrence was then without a parallel. 

 More recent observations of similar phenomena leave no 

 room to question the historian's veracity. 



2. The head of the great comet of 3S9 A.D., according 

 to the writers of that period, was "composed of several 

 small stars " (Hind's " Comets," p. 1031. 



3. On June 27, a.d. 416, two comets appeared in the 

 constellation Hercules, and pursued nearly the same ap- 

 parent path. Probably at a former epoch the pair had 

 constituted a single comet.f 



4. On Aug. 4, 813, " a comet was seen which resembled 

 two moons joined together." They subsequently sepa- 

 rated, the fragments assuming different forms J 



5. The Chinese annals record the appearance of three 

 comets — one large and two smaller ones— at the same 

 time in the year S96 of our era. " They travelled together 

 for three days. The little ones disappeared first, and then 

 the large one."§ The bodies were probably fragments of 

 a large comet which, on approaching the sun, had been 

 separated into parts a short time previous to the date of 

 their discovery. 



6. The third comet of 1618. — The great comet of 1618 

 exhibited decided symptoms of disintegration. When first 

 observed (on November 30), its appearance was that of a 

 lucid and nearly spherical mass. On the eighth day the 

 process of division was distinctly noticed, and on the 20th 

 of December it resembled a cluster of small stars. 1! 



7. The comet of 1661. — The elements of the comets of 

 1532 and 1661 have a remarkable resemblance, and pre- 

 vious to the year 1790 astronomers regarded the bodies as 

 identical. The similarity of the elements is seen at a 

 glance in the following table : — 



Comet of 1532. Comet of 1661. 

 Longitude of Perihelion . iii°4S' . 115° 16' 

 Longitude of Asc. Node . 87 23 . Si 54 



Inclination 32 36 , 33 I 



I'erihehon Distance . . 0-5192 . 04427 

 Motion Direct . Direct 



The elements of the former are by Olbers ; those of the 

 latter by Mechain. The return of the comet about 1790, 

 though generally expected, was looked for in vain. As a 

 possible explanation of this fact it is interesting to recur 

 to an almost forgotten statement of Hevelius. This as- 

 tronomer observed in the comet of 1661 an apparent 

 breaking up of the body into separate fragments. IT The 

 case may be analogous to that of Eriela's comet. 



8. The identity of the comets of 1866 and 1366, first 

 suggested by Prof. H. A. Newton, is now unquestioned. 

 The existence, then, of a meteoric swarm, moving in the 

 same track, is not the only evidence of the original comet's 

 partial dissolution. The comet of 1866 was invisible to 

 the naked eye ; that of 1366, seen under nearly similar 



• " QuKst., N.il.," lib. vii., cap. .\vi. 

 t Chambers's *' Descr. Astr.," p, 374. 

 t Ibid. p. 383, 

 5 Ibid, p, 388. 



II Hevelius, " Cometographia," p. 341. See also Grant's "History of 

 Physical Astronomy," p. 302. 

 *t\ " Cometo^aphia," p, 417. 



