498 



NA TURE 



[March 23, 1899 



ihe real one. Schiaparelli detected the dynamical explanation 

 of the fact that the swarm is leni;thened out like a stream along a 

 portion of Adams's orbit. And Leverrier adduced evidence that 

 the Leonids have been less than eighteen centuries within the 

 solar system : that in fact they were diverted into their present 

 elliptic orbit at the end of February or beginning of March in 

 the year A.n. 126, in consequence of having then pas.sed, while 

 still a compact cluster, close to the planet Uranus. Adams 

 further pointed out that there is a 'Comet moving nearly in their 

 track. 



These were great achievements ; of which the most noteworthy 

 is the great discovery made by I'rof. .\dams when he determined 

 definitely the real orbit in which these bodies move. This he 

 .accomplished by computing the perturbations which would be 

 suft'ered in each of the five possible orbits, and comparing the 

 calculated amount of the .shift of the nodes with that which had 

 been obtained by comparing the ancient with recent observ- 

 ations.. 



The main swarm of Leonids is again returning. A shower of 

 several hundreds of meteors, produced by the extreme front of 

 the ortho-stream, was observetl last November in .\merica. 

 Still greater showers may be expected this year and next year, 

 and perhaps a considerable display in the year following ; and 

 it is eminently desirable that this opportunity of increasing our 

 knowledge in this entirely new brancn]of astronomy shall not be 

 lost. It is the second occasion when astronomers have been 

 able to foresee when the opportunity is about lo present itself. 



In 1866, the great object was to ascertain the orbit. To 

 determine this, what was wanted was the average amount of the 

 perturbations, and it was this average which Adams computed. 

 But to make a further advance— to explore more fully the past 

 history of the Leonids, or their present condition, or to predict 

 the future— a more intimate acquaintance with the perturbations 

 is essential. Now perturbations reach each meteor individually. 

 They dift'er from one revolution to another, and within each 

 revolution they variously aftect the meteors that occupy difTerent 

 stations along the stream. 



The present investigation was entered on as a commencement 

 of the more searching inquiry indicated above. The stream is 

 regarded as divided into segments of such moderate length that 

 the perturbations which operate on the meteors occupying any 

 one of them may be regarded as sensibly the same. One of 

 these segments is selected— that through which the Earth passed 

 in 1866 — and the actual perturbations to which the elements of 

 its orbit are being subjected throughout an entire revolution, 

 have been computed by the method of mechanical quadratures. 

 The revolution extends from 1 866 November 13, when the Earth 

 passed through this segment of the stream, till 1900 January 

 27, when the sime segment will return to the intersection of 

 Ihe meteoric orbit with the Earth's orbit. 



The inquiry has already led to remarkable results. During 

 this revolution an entirely abnormal amount of perturbation has 

 acted on the meteors in the selected .segment of the stream. This 

 perturbation has been produced chiefly by the attraction 

 exercised by the great planets Jupiter and Saturn, and its unusual 

 amount has been occasioned by a near approach of Saturn w hen 

 that segment of the stream, for which the calculations were made, 

 was on its outward journey, and a still more close approach of 

 Jupiter, when the meteors were on their homeward journey. 

 These events have resulted in such a perturbation of the orbit, 

 that the shift of its node during this revolution has had more 

 than 3J times its average amount, and that the periodic time 

 has become augmented by as much as i of a year. 



This last perturbation will have a remarkable effect on the 

 future history of this segment of the stream, unless it is com- 

 (x-nsated by what occurs elsewhere or in subsequent revolutions. 

 It indicates, too, that whatever portion of the stream has been 

 most perturbed in this revolution is falling back towards the 

 parts behind and retreating from the portions in front ; thus 

 introducing a new inequality of distribution of density along the 

 .stream, superadded upon whatever inequalities of a like kind 

 may have existed jjreviously. Thus some parts of the stream 

 are becoming unduly crowded with meteors. Others of the 

 perturbations indicate that in this remarkable revolution a new 

 sinuosity of sensible amount is being set up in the stream. These 

 effects have been made conspicuous by the fortunate circum- 

 stance that the revolution for which the calculations have been 

 m.ade has happened to be one in which the perturbing forces 

 have attained an intensity far exceeding the average. 



The information supplied by this inquiry in regard to the 



NO. 1534, VOL. 59] 



lime when the I.'onid shower of next November may be ex- 

 pected IS considerable, but fai from complete. It may be stated 

 as follows : — .\t the epoch 1899, Novendjer 15, the longitude 

 of the node of the orbit for which the calculations have been 

 made will be 53' 41' 7, a position which the earth will reach on 

 1899 Noveinber Ijd. i8h. It is probable, therefore, that the 

 middle of the shower of the present year (1S99) will occur nearly 

 at this time, since the segment of the stream, for which our calcu- 

 lations have been made, is situated in the stream less than three 

 months' journey of the meteors behind the segment which the 

 Earth will encounter next November. This conclusion, how- 

 ever, rests on two assumptions : (i) That the two segments 

 were, in 1866, moving in orbits that did not much differ ; (2) 

 That the perturbations which these segments have since suf- 

 fered have not much differed, both assumptions are probable, 

 but unfortunately neither is certain ; so that the prediction can 

 only be offered with reservation. If the shower occurs at the 

 time anticipated, it will be visible from both Europe and 

 America. 



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LOCAL AUTHORITIES FOR SCIENCE AND 

 ART INSTRUCTION. 

 'T'HE Directory issued by.the Department of Science and Art 

 •*■ in 1897, contained a section which has since become widely 

 known, .and will probably take a prominent place in educational 

 politics for some tinu-. The new paragraph — referred to as Clause 



