82 



NATURE 



[November 22,, 1899 



perihelion (January ii), while the shower of 1868 

 appeared nearly three years after the return of the comet. 

 The latter object probably reached perihelion last spring 

 {though it was not observed anywhere), and we are 

 clearly entitled to expect from the great extension of the 

 meteor stream visibly encountered from 1866 to 1868 

 that exhibitions of the finest kind will be presented in 

 the two ensuing years. Whether or not the phenomenon 

 will be favourably perceptible in England is uncertain, 

 but it ought to be seen in one of the two years. 



In 1833 there was a magnificent display. In 1866 

 the earth passed through a section of the orbit three 

 months in advance of the part we encountered in 1833. 

 There was a very rich shower in 1866, but it was nothing 

 like the preceding one. In 1899 the earth intersected 

 a region of the stream six months in advance of that of 

 1833, and where the meteors are thinly distributed. 

 Everything supports the view that observers will not 

 watch in vain for these meteors in 1900 and 1901. 



When we consider the circumstances affecting the 

 visibility of the Leonids, we must readily concede that it 

 will often evade notice at a given place. In England, 

 November nights are rarely clear and clouds may hide 

 the meteors, or the earth may traverse the swarm at 

 some time during the 15 hours in a day when it possibly 

 could be seen, for from 7 a.m. to after 10 p.m. either day- 

 light or an invisible radiant places it beyond reach. But 

 many of us will hope to find compensation for the dis- 

 appointments of recent years in observing a brilliant 

 return of the meteors in one of the two ensuing years, 

 and certainly before the denser region of the stream gets 

 too far on its outward journey to aphelion. 



W. F. Denning. 



Cause of the Non-appearance of the Shower. 



None of the Leonid meteors are visible until and unless 

 some out of their vast number chance to plunge into our atmo- 

 sphere and are extinguished after a second or two of intense 

 brilliance. We cannot accordingly follow their motions by 

 observations in the open sky, and can only tell where they are 

 when we can compute where they must be. This has become 

 possible with reference to station A in the stream, that portion 

 through which the earth passed in 1866, and of which Adams 

 ■determined the osculating ellipse as it existed in that year. 

 Any change either in form or position which it has since under- 

 gone has been due to perturbations. The meteors occupying 

 that portion of the stream have nearly completed another 

 revolution since 1866, November 13. The perturbations they have 

 suffered in the latter part of their course have been computed 

 in Germany by Dr. Berberich, and the perturbations 

 ■over the whole of the revolution have been computed in this 

 country by Dr. Downing and the present writer, with the aid of 

 the skilled computers of the Nautical Almanac, and at the ex- 

 pense of the Royal Society. These more full computations 

 enable us to follow all the motions of this portion of the stream. 

 It will reach its descending node, where it comes nearest to the 

 earth's orbit, on the 27th of next January, and is accordingly at 

 present advancing towards the earth, along an osculating 

 ellipse of which the present form and position can be determined. 

 This has been done, and it has been thus ascertained that the 

 earth passed the descending node of this orbit last Thursday 

 morning at about 6 a.m. In 1866 this orbit intersected the 

 earth's orbit, but unusually intense perturbations have since 

 acted on it, and have so shifted its position that the point when 

 it pierced the plane of the earth's orbit last Thursday, and which 

 we may call point P, lay at a disrance from the earth towards 

 the sun which was o'oi4i of the mean distance of the sun, that 

 is, it lay about five times farther from the earth than the moon 

 is. A subsidiary investigation, which will shortly be published, 

 makes it almost certain that the point P indicated above is 

 situated within the stream which was passing the earth last Thurs- 

 day. This is the only point in the stream which was passing us 

 last Thursday of which we actually know the position ; and it 

 was at the great distance from us which is above stated. 



Now comes in another consideration. A separate dynamical 

 investigation into the conditions under which the Leonids were 

 drawn into the solar system by Uranus, has shown that when 



that planet advanced along his orbit and left them behind, they 

 found themselves moving nearly with the same speed and nearly 

 in the same direction, but not quite. They were in fact scattered 

 over a very small cone of dispersion. This occasioned small 

 differences to exist between the vast elliptic orbits round the 

 sun, upon which they then entered. Some of the meteors found 

 themselves in planes slightly more inclined to the ecliptic than 

 others, some started along ellipses of slightly greater ellipticity, 

 and so on ; but all when they had travelled along the inward 

 part of their new* elliptic journeys would cross the plane 

 of Uranus's orbit (which is nearly the plane of the 

 earth's orbit) at points which lay along the line of 

 nodes, measured in the plane of Uranus's orbit, a line 

 which nearly coincided with the radius of the earth's orbit, 

 which lay along the line of nodes in the plane of the ecliptic. 

 Hence the stream became a ribbon-shaped stream at its 

 descending node, where the earth encounters it, the width of 

 the ribbon lying very nearly along one of the radii of the earth's 

 orbit. The position of this ribbon has been somewhat altered 

 by the perturbations to which it has been exposed during the 

 seventeen centuries and threequarters which have since elapsed. 

 Its width accordingly no longer lies quite perpendicular to the 

 earth's orbit. We know that the stream has this ribbon shape, 

 but we do not know its width further than that it is considerable, 

 nor do we know where in the width of the ribbon the point P 

 lies whose position we have been able to determine. That we 

 have not had one of the great Leonid showers this year con- 

 clusively proves that the part of the width of the ribbon which 

 lies outside the point P has not been able to reach the whole 

 way out to the earth's orbit — a distance of about 1,300,000 

 miles. G. Johnstone Stoney. 



Greenwich Observations. 

 The Astronomer Royal reported to the Times on the i6th 

 inst. , that the preparations made at Greenwich for observing 

 the Leonid meteors were rendered abortive by cloud and fog on 

 the nights of November 14-15 and 15-16. During a short 

 break in the clouds on the morning of the i6th only 16 Leonids 

 were noted (by four observers) in 42 min. from 5h. 34min. to 

 6h. i6min. A.M. November 16. No photographs could be 

 obtained. 



Report from the Solar Physics Observatory. 



To take advantage of the meteor shower that was expected at 

 the earlier part of last week, the whole staff of the Solar Physics 

 Observatory took part in a carefully-prepared programme. The 

 observers were divided up as follows : The six-inch Dallmeyer 

 camera for photographing the radiant point, and a siderostat 

 with three small cameras mounted on its polar axis for obtaining 

 spectra were worked by Dr. Lockyerand Mr. Howard Payn, who 

 was a volunteer. Mr. Fowler took charge of an integrating 

 spectroscope and a small visual spectroscope mounted equa- 

 torially to examine bright trails. The large 6-inch prismatic 

 camera was used by Mr. Baxandall and Mr. Shackleton on 

 alternate nights, while a 9-inch prismatic reflector and another 

 battery of small cameras was worked by Mr, Butler and Mr. 

 James. 



On all the four nights (9 p.m. to 6 a. m.) during which a watch 

 was kept, the weather was very unfavourable, and it was only for 

 shott periods of time on Wednesday and Thursday that a glimpse 

 of the sky was at all possible. In fact, fog and cloud seemed to al- 

 ternate or combine at the expense of a clear sky. To take advan- 

 tage of a clear sky at some distance from the observatory, such as 

 at Hampstead, several volunteers took up their positions there 

 with advantage. At the observatory itself no photograph of any 

 meteor trail or spectrum was obtained, and it was practically 

 only for a short period on Thursday morning that plates were 

 actually exposed with any prospect of success. Eye observations 

 indicated, however, that if the shower had arrived on Wednes- 

 day or Thursday, at least some trace of its presence would have 

 been seen during the period of observation, in spite of the fog, 

 if the display had attained anything like its grandeur of 1866. 



Tuesday night was apparently very clear at Hampstead, and 

 one of our keen amateurs reported that between 10 p.m. and 

 4 a.m. the next morning, there was an absolute dearth of 

 meteors. Another observer on Banstead Downs also saw no 

 signs of the shower, for between 2.28 a.m. and 3 a.m. on the 

 Wednesday morning, he counted only ten meteors, and these 

 might not all have been Leonids. Brighton had a clear sky on 

 the morning of Wednesday, and an observer there who watched 



NO. 1569, VOL. 61] 



