NA TURE 



[October 28, 1897 



branch is such an extremely difficult one, embracing, as 

 it does, some thousands of streams which exhibit many 

 different peculiarities, that in order to grapple with the 

 subject successfullV he must make it his constant care 

 ^d the object of his earnest efforts and thoughts during 

 many years. 



Fortunately the Leonids are to be classed amongst that 

 description of meteors comparatively easy to observe and 

 record. They leave streaks for a second or two, and 

 from these the directions are to be determined with great 

 facility and precision. It is also a fortunate circumstance 

 that the radiant point is surrounded by the well-known 

 stars in the Sickle of Leo. The lines of flight may 

 therefore be readily carried back in the correct direc- 

 tions by projecting a straight wand upon the streaks, and 

 noting their points of convergence relatively to the stars 

 named. 



The writer has usually found the radiant very definitely 

 and sharply defined, and it can be readily fixed to within 

 2° of probable error. But naked-eye observation is 

 capable of much more accurate results than this, if, 

 during a pretty active return of the shower, the observer 

 will independently fix the radiant during, say, successive 

 half-hours of the night ; he will in this way get eight, ten, 

 or twelve positions, from which he may derive the mean 

 place of the radiant to within about |° of error. 



Tlie Leonid radiant is sometimes described as very 

 diffuse ; but this is a false effect brought about by two 

 circumstances which, if properly allowed for, would leave 

 a very definite and satisfactoiy position. One cause of 

 its apparent diffusion is that meteors are attributed to it 

 which really belong to the minor showers in Leo and the • 

 surrounding region, of which quite a large number exist. 

 They display similar visible characteristics to the Leonids, 

 and can only be dissociated from them by the exercise 

 of extreme care in noting their directions of flight. In 

 Popular Astronomy, vol. i. p. 298, I gave a list of sixty- 

 eight meteoric radiants situated in various parts of the 

 heavens, and active during the period November 10-15 '■> 

 and in The Observatory, vol. xx. p. 306, a table of 

 seventy-two circum- Leonid showers was published. 

 Those which chiefly affect the determination of the 

 Leonid radiant are placed near 8 and i Cancri, yx and ^ 

 Ursai Majoris, X H)drre and tt, X, r and ^3 Leonis. 

 The meteors are swift, and usually leave streaks. An- 

 other contributing feature to dispersed radiation is found 

 in the unavoidable errors of observation. Great care and 

 habitual practice can, however, reduce these to small 

 limits, and it will be found that the radiants derived from 

 accurate materials will be pretty sharply defined. 



The probable error in the case of different observers 

 must, however, vary to a considerable degree, for prac- 

 tice cannot equally eliminate inaccuracy from amongst 

 them all. In catching and retaining correct impressions 

 of meteor flights, natural aptitude exercises an important 

 influence. It is like a game of skill depending upon the 

 eye, judgment and quickness in execution. Really iG.\v 

 will excel, while many will only attain mediocrity, and 

 some must altogether fail to acquire the desirable 

 proficiency, even after years of experience. 



The horary rate of appearance of Leonids cannot be 

 exactly determined unless the contemporary showers are 

 considered, and their meteors separated from the true 

 Leonids. Many observers count every meteor proceeding 

 from the general direction of Leo as necessarily a Leonid, 

 and thus the horary number is exaggerated. If an in- 

 experienced observer gives 20 as the number of Leonids 

 seen in an hour, the fair inference is that not more than 

 14 or 15 of them were true members of that system. 

 •During very strong returns of the shower this point may, 

 however, be disregarded, for the minor streams can then 

 exercise very little relative influence on the results, and are 

 virtually obliterated by the superabundance of Leonids. 



One new feature to be atteinpted during the ensuing 



NO. 1 46 1, VOL. 56] 



return of the Leonids is to photograph the meteor group 

 of November 1866 in space, and an excellent ephemeris 

 of its nearly stationary position in Libra and south- 

 eastern limits of Virgo, during the first four months 

 of 1897, was given in Monthly Notices, Ivii. p. 70-2. 

 Some people will regard the idea as little more feasible 

 than opening a correspondence with the inhabitants of 

 Mars, and certainly there appears very slender prospect 

 of its successful realisation. The experiment ought, 

 however, to be tried. Let us support e\ ery project which 

 has a possible side to it, for it is quite clear that many 

 things deemed beyond our reach are capable of attain- 

 ment by persevering efforts and proper means. Novel 

 attempts of this kind, if seemingly chimerical, should not 

 be hastily condemned or necessarily considered as vain 

 labour. Mr. Roberts's photographic search for a trans- 

 Neptunian planet was a novelty, and it proved vain 

 labour ; but who will say that it ought not to have been 

 undertaken 1 The same may be said of Mr. Barnard's 

 similar search for a satellite to the moon. To look for a 

 fifth satellite of Jupiter was decidedly a novelty in these 

 modern times, and yet it proved productive. Let, then, 

 new researches like these have our encouragement ; for if 

 they do not always succeed, they stimulate our interest 

 and enthusiasm, and make the science more attractive by 

 imparting to it a welcome freshness and, perhaps, a touch 

 of romance. 



As to the practical aspect of the question, it is fair to 

 conclude that the Leonid group of 1866 is too faint an 

 object to be ever impressed on a photographic plate, 

 especially when its distance is so great as during the 

 past spring, for on March i this was equivalent to 

 800,000,000 miles, and not far short of the mean distance 

 of Saturn ! In the great meteor storm of November 27, 

 1885, when the meteors were more thickly congregated 

 than in the Leonid shower of November 13, 1866, Prof. 

 Newton computed that •' the space in the meteroid 

 group corresponding to each single visible meteor was in 

 the densest portion of the group, a cube whose edge is 32'8 

 kilometres or20'4 miles." This means one small pebble 

 in twenty miles of space ! The degree of illuminating 

 power exhibited by a group of these bodies, separated by 

 such distances, must be infinitesimally small. If any one 

 were to attempt to photograph Tempel's comet (1866 I.), 

 on its return journey, the chances of success would be 

 far greater, for though the comet has still to run eighteen 

 months before reaching perihelion, it is nearer to us 

 than the meteor group of 1866, and must be infinitely 

 brighter, as it doubtless represents the richest part of 

 the stream. We must remember that Tempel's comet 

 passed its perihelion on January 11, 1866, while the 

 meteor-group reached it ten months afterwards ; and it 

 is quite fair to suppose that the meteoric train of the 

 comet, at a distance of some hundreds of millions of 

 miles from the nucleus, must be relatively tenuous as 

 compared with that part in the immediate wake of the 

 comet. The meteors may not, however, show a regular 

 decrease in numbers according to distance from their 

 derivative comet, but may probably consist of a series of 

 groups. There is every reason to believe that disruptions 

 of a violent character affect the physical character of 

 comets, and this was well exemplified in Brooks's comet 

 (1889 v.), vi'sible, in 1896, at its second observed return, 

 which was seen separated into five portions on August 

 r, 1889. There is, however, every probability that the 

 meteor cluster of 1866 is some hundreds of times fainter 

 than Tempel's comet ; yet even the latter was not visible 

 to the naked eye in December 1865, or January 1866, 

 and indeed the object was only followed for a month in 

 telescopes. It might be a good plan to endeavour to 

 photograph the comet first, and then fish for its associated 

 meteor-stream ; for the easier objects are sometimes 

 capable of leading us up to the discovery of the more 

 difficult ones. W. F. Denning. 



