194 



NATURE 



{August 23, 1888 



But the epoch of 1866 is perhaps the most eventful and 

 interesting of all in the history of this notable group. 

 Signor Schiaparelli, of Milan, in the course of some ob- 

 servations of the Perseids, was led to take up the investi- 

 gation of the theory of shooting-stars. Cautiously sifting 

 the available materials, and forming deductions from 

 facts indicated by the best authorities on the subject, he 

 was induced to the belief that meteors were small par- 

 ticles composing cosmical clouds. These clouds were, 

 by the action of gravitation, spread out into streams, and 

 their orbits formed, like those of comets, elongated conic 

 sections. From a method explained by Prof. Erman, he 

 computed the orbital elements of the August meteors 

 and of certain other streams, and, comparing them with 

 the orbits of comets, discovered two remarkable coin- 

 cidences between the system of Perseids and Comet 1 1 1. 



1862, and the Leonids and Comet I. 1866. In each case 

 the paths of the meteor group and comet were identical, 

 and every circumstance favoured the inference that the 

 two phenomena were physically identical, the meteors 

 forming the dispersed material of the comet. The period 

 of the Leonids (November 13), viz. 33} years, agreed 

 precisely with that of their supposed parent comet. The 

 period of the August display, however, remained doubt- 

 ful, the ellipse being more elongated ; but Schiaparelli 

 adopted a cycle of rather more than 100 years, as best 

 satisfying the observations, though the exact period is 

 still doubtful. 



Computation showed that the radiant point of meteoric 

 particles following the track of Comet III. 1862 would 

 be seen, on August 10, at R.A. 43 , Decl. 57*° N. In 



1863, on August 10, Prof. Herschel had observed the 

 meteors, and fixed their radiant at R.A. 44°, Decl. 56° N., 

 a wonderfully close agreement, considering the difficul- 

 ties attached to such observations. This, and other co- 

 incidences of orbit, removed all doubts as to the affinity 

 of meteors and comets ; and later evidence, especially 

 that afforded by Biela's comet and the splendid meteor- 

 showers of November 27, 1872 and 1885, has afforded con- 

 vincing proofs as to the validity of the theory enunciated 

 by the Italian astronomer. 



Some interesting features in connection with the 

 August meteors still, however, awaited further investiga- 

 tion. The visible duration of the shower was unknown. 

 The radiant was thought to be diffused over a region 

 extending from Perseus to Cassiopceia. Mr. R. P. Greg, 

 in his "Table of Radiants" {Matithly Notices*, 1872, 

 p. 353), places it over the area from R.A. 50°-25°, Decl. 

 44 N., to R.A. 5o°-65°, Decl. 56° N. ; and Serpieri gave 

 R.A. 5o°-3o°, Decl. 49°-64°. Mr. J. E. Clark, in 1874, 

 undertook the projection of the tracks of about 2000 

 Perseids described in the "Luminous Meteor Reports" 

 of the British Association, with the object of detecting 

 motion in the radiant centre on successive days or hours 

 of the night, but without definite success, though the 

 observations suggested a progressive motion on succeed- 

 ing nights similar to that noticed by Prof. Twining in 

 1859. In 1877 the shower was watched by the writer at 

 Bristol on several nights, and the radiant was distinctly 

 seen to take up a fresh position with every change of 

 date. It moved from R.A. 40°, Decl. 56°, on August 5, 

 to R.A. 60°, Decl. 59 N., on August 16. The fact was first 

 announced in Nature for August 30, 1 877 (vol. xvi. p. 362), 

 and many observations in subsequent years at the same 

 station have fully confirmed the shifting of the radiant, 

 and indicated the long duration of the shower. In the 

 following table will be seen the position of the radiant at 

 intervals of five days : — 



July 8... 



13- 

 19... 

 23... 

 28... 



The whole duration extends, very probably, over the 

 forty-five days from July 8 to August 22, and in the 

 interval the radiant moves from 3 + 49 to 77 + 57°. 



This cluster is evidently one of enormous width, and 

 has doubtless undergone distortion by the effect of 

 planetary perturbation. Some interesting facts in con- 

 nection with this and other cometary meteor systems 

 will be found in the Sidereal Messenger for April and 

 May 1886. With regard to the August meteor-shower, 

 it appears that a certain change in the position of the 

 radiant ought theoretically to occur every night, but the 

 observed displacement does not well accord with compu- 

 tation. On July 26 the Perseid radiant is about 4°, and 

 on August 19 about 9 , from the radiant of its derivative 

 comet (III. 1862) ; and these differences are doubtless to 

 be referred to the disturbances exercised upon the ori- 

 ginal stream by the attraction of the earth. At every 

 return of the group a vast number of the particles must 

 obviously pass very near to us without being dissipated 

 by the action of our atmosphere, and the paths of these 

 will be affected to an extent that must alter the elements 

 of their orbits. 



Though the period of the August meteors has not yet 

 been precisely ascertained, there is no question that the 

 shower exhibits fluctuations from year to year as regards 

 intensity, and that, like the two great systems of Novem- 

 ber, a certain cycle regulates its most brilliant displays. 

 Future observations will determine the precise form of 

 the orbit. The return of Comet III. 1862, or a recur- 

 rence of the very rich shower of August 1863, will decide 

 the matter, but as the orbit is one of considerable eccen- 

 tricity, several generations may yet elapse before the 

 period is accurately ascertained. It is certain that many 

 of the supposed variations in the perennial intensity of 

 the display are more apparent than real, because the suc- 

 cessive returns are witnessed under different conditions. 

 Cloudy or misty weather sometimes interrupts observa- 

 tion ; moonlight offers another impediment ; occasionally, 

 also, the maximum is attained in daylight, and passes 

 unheeded. The same observer is not always enabled to 

 maintain an outlook from positions equally favourable ; 

 and there are other circumstances which, with those men- 

 tioned, prove the difficulty of securing a series of observa- 

 tions fairly comparable with each other. Usually about 

 40 or 50 meteors per hour may be counted by one 

 observer before midnight on August 10, but in the early 

 morning hours of August 1 1 as many as 80 or 90, 

 perhaps more, will be seen, as the radiant is then 

 higher and better placed for the visible distribution of 

 its meteors. 



" The August meteors," though a general term capable 

 of being applied to any showers observed in the month of 

 August, is commonly employed in special reference to 

 the Perseids of August 10. There are large numbers of 

 minor displays visible in the same month, the radiant 

 points of which are scattered profusely over the firma- 

 ment. There are certainly more than 100 showers in 

 contemporaneous action with the Perseids, and many of 

 these are now pretty well known, a mass of observations 

 having accumulated for this particular epoch. 



In the present year the great August shower has not 

 been especially brilliant, though many of its meteors 

 have appeared under their customary aspect. At Bristol, 

 on August 2, 42 shooting-stars were counted during the 

 2^ hours between loh. 50m. and J3I1. 21m., and 14 of 

 these were Perseids from a centre at 35 -f 54". On 

 August 5, 31 meteors were seen in a similar interval, 

 including 11 Perseids. On August 8, in 3 hours from 

 ioh. to i3h., 36 meteors were observed, and among these 

 were 20 Perseids. The radiant, both on the 2nd and 

 5th, seemed to be at 42 -+- 57". The few subsequent nights 

 were overcast, but on the 13th a clear sky permitted 

 watching, and during the 34 hours from ioh. to 13b. 30m. 

 49 meteors were seen, of which 13 were Perseids from a 



