3G 



KNOWLEDGE. 



[Febrhaey 1, 1894. 



But in Prof. Barnard's photogi'aph of Brooks' comet, 

 taken on the 21st of October, the general curvature of the 

 tail is for some distance from the nucleus concave towards 

 the part of space to which the comet was moving, while 

 the fainter and more distant parts of the tail exhibit a 

 general curvature which is concave towards the region 

 from which the comet was moving. 



If the reader will carefully compare the stars shown 

 upon the photographs taken upon the 21st and 22nd of 





Sketch bv ilr. .1 ( 



nx Brett ol Comet (I.) is.si, 

 171i. Groenwii'li nicau time. 



made 22nd Oetober 



October, he wiU see that in the twenty-four hours which 

 elapsed between the exposure of the two photographs the 

 nucleus of the comet had moved in a northerly or north 

 by east direction through a distance which corresponds to 

 about seven-eighths of an inch on the scale of our plate. 

 The two small stars which are shown on the photograph 

 of the 22nd of October (the one to the east and the other 

 to the west, at a distance of about one-eighth of an inch 

 from the centre of the nucleus) will be foimd ou the 

 photograph of the 21st of October at a distance of about 

 seven-eighths of an inch from the nucleus in a direction 

 nearly coinciding with the line joining the nucleus with 

 the upper left-hand corner of the plate. 



If the matter of the comet's tail was deflected 

 in its course from the nucleus by an encounter 

 with a resisting medium which was stationary in 

 space we should expect to find evidence of resist- 

 ance in a direction contrary to the direction of 

 the comet's apparent motion ; but though the 

 advancing side of the tail is notched and sharjily 

 defined in the lower half of the tail, the opposite 

 or following side is most sharply defined in the 

 upper and fainter parts of the tail, and the 

 following side of the tail is more sharply defined 

 than the precedmg side in the photograph taken 

 on the 22nd of October. 



As in the photographs of Swift's comet, published in the 

 December number of Knowledge for 1892, there is striking 

 evidence in these photographs of Bi-ooks' comet that the 

 matter of the tail was not driven away in a uniform 

 stream from the nucleus. Prof. Barnard has noticed the 

 detached cloud completely separated from the end of the 

 tail on the 22nd of October, and the rest of the tail in the 



photograph taken on the 22nd of October is broken into 

 fragments, indicating short spasmodic outbursts, during 

 which matter must have been driven away in considerable 

 quantities from the nucleus, followed by quieter intervals. 

 ■That these outbursts were of comparatively short duration 

 is, I think, proved by the slight curvature of the tail, 

 which indicates that the motion of the nucleus was not 

 considerable during the time occupied by the passage of 

 matter from the nucleus to the end of the tail. On the 

 other hand, the velocity of the matter of the tail 

 ^ away from -the nucleus does not seem to have 

 '\ lieen sufficiently great to disturb the definition of 

 the notches on the edge of the tail in the thirty- 

 live minutes during which the, photograph was 

 exposed, on the 21st of October. 



Such rapid changes in the amoimt of matter 

 driven away from the nucleus would seem to point 

 to an irregular evolution of energy, such as might 

 be caused by the passage of the nucleus through 

 an irregularly distributed resisting medium, or 

 through swarms of meteors, rather than to the 

 evaporation of matter due to a steady increase of 

 heat on approaching the sun. 



The notches and irregularities in the edge of 

 the tail, as well as the branching structures in 

 the photograph of the 21st of October, seem to 

 me to point to outrushes of matter from the 

 nucleus through a resisting medium, in different 

 directions, which outrushing matter has afterwards 

 been driven away from the sun, rather than to 

 a disturbance of the regular form of cometary 

 tail due to an encounter of the matter in the tail 

 with a resisting medium at a distance from the 

 nucleus. 



The general form of comets' taOs, and the way in 

 which they develop in size as comets approach the 

 Sim, and diminish again as they recede fi'om their perihelion 

 position, points to the conclusion that the development of 

 cometary tails is chiefly due to the action of the sun's heat ; 

 but the irregularities of form we are discussing point to more 

 sudden changes, causing an irregular development of heat 

 in portions of the nucleus, which gives rise to radial out- 

 rushes of gaseous matter in various directions. Such 

 irregularities in the evolution of gas from the nucleus 

 might be caused by collisions or internal disturbances in a 

 loosely-piled group of stones, or cluster of meteors, ou 

 approaching perihelion ; or it might be due to the rapid 

 passage of external groups of meteors, or clouds of 



i 



Comet (J) 1882, from a photograpli taken 



Observatory. (Undated.) 



llo. 



meteoric dust, into a loosely-packed gaseous envelope 

 surrounding the cometary nucleus. 



Though the earth does not encounter groups of meteors 

 sufficiently dense to materially raise the temperature of 

 large regions of its upper atmosphere, it does not necessarily 

 follow that denser groups of meteoric stones may not exist 

 in other parts of the solar system, and it must be admitted 



