May 1, 1891.] 



KNOWLEDGE 



87 



or the Northern Crown ; in Corona Austrahs in the southern 

 hemisphere ; and also among fainter stars visible to the 

 naked eye, or w-ith an opera-glass. 



But it is among the still fainter stars — those visible only 

 with a telescope or revealed by photography — that we find 

 the most striking examples of this stream-forming ten- 

 dency. In these cases the small stars composing the 

 streams are comparatively close together — at least, ap- 

 parently so — and for this reason the evidence in favour of 

 a real physical connection is proportionately stronger. 

 Webb says* : " A little n. ji. /x Sagittarii xviih. 57m., 

 S. 18° 50' is a spot referred to by Secchi as exemplifying 

 in a high degree the marvellous structure which the great 

 achromatic at Eome shows in the Galaxy. The remarks 

 of this accomplished astronomer on the successive layers 

 of stars are very curious : first he finds large stars and 

 lucid clusters ; then a layer of smaller stars, certainly 

 below 12 mag. ; then a nebulous stratum with occasional 

 openings. But what he says startled him, and all to 

 whom he showed it, was the regular disposition of the 

 larger stars in figures ' si geometriques qu'il est impossible 

 de les croire accidentelles. La plus grande partie sont 

 comme des arc de spirale ; on pent compter jusqu'a 10 ou 

 12 ctoiles de la 9me. a la lOme. grandeur. . . . Se sui- 

 vant sur une meme courbe comme les grains de chapelet ; 

 quelquefois elles forment des rayons qui semblent diverger 

 d'un centre commun, et co qui est bien singulier, on voit 

 d'ordmaire que, soit au centre des rayons, soit au com- 

 mencement de la branche de la courbe, on trouve une 

 etoile plus grande et rouge. II est impossible de croire 

 que telle distribution soit accidentelle.' " 



I have already noticed that on the Paris stellar photo- 

 graphs many cases may be seen of three or more stars 

 placed in a straight line, or nearly so. Sometimes a 

 comparatively bright star seems to draw a train of fainter 

 stars after it, like the tail of a comet, and occasionally a 

 stream of stars of nearly equal brightness may be traced 

 for some distance from their source. In the photograph 

 of the cluster 38 Messier, this stream-formation is well 

 marked among the brighter stars. Webb describes it as 

 " a noble cluster arranged as an oblique cross. "+ 



Observing with a 3-inch telescope in India, in July 

 187-1, I noticed a beautiful cluster of stars about ■L° north 

 of \ and V Scorpii, resembling, in shape, a bird's foot, with 

 remarkable streams of stars. This cluster is visible to the 

 naked eye as a star of 5 or 5s magnitude. 



Sir WiUiani Herschel, speaking of the compressed 

 cluster H. vi. 25 in Perseus, says, " the larger stars arc 

 arranged in luaes like interwoven letters"; and, Webb 

 Bays, " it is beautifully bordered by a brighter fore- 

 shortened pentagon." 



From the close proximity of the component stars — of 

 some at least of these clusters — the reality of a physical 

 connection between them seems beyond dispute, and from 

 analogy we may conclude, I think, that " streams " and 

 " sprays " of stars in otlier portions of the heavens are, in 

 some cases at least, due to a real and not merely an 

 apparent connection. 



From a telescopic examination of the Milky Way, 

 Professor Holden finds numerous star - streams, and 

 also arrangements forming "small definite ellipses" of 

 stars, often all of the same size. ... In certain parts of 

 the sky, the arrangement is so intricate that no single 

 pattern can be discovered. In most regions a little atten- 

 tion will show that there are several patterns, one for 



* Celestial Ohjects, Fourth Kdition, p. 3S5, foot-note. 

 i Ibid., p. '2il. 



each of the fainter magnitudes of stars " (Monthly y(jtices, 

 E.A.S., Dec. 1889). 



In a paper in the Monthly Xotices, R.A.S., for April 

 1890, Mr. Backhouse calls attention to the " straight lines 

 and parallel arrangements of pairs, lines, and bauds of 

 stars, and also of irresolvable wisps " observed by him in 

 a portion of the Milky Way included between the .stars 15, 

 13, 8 Monocerotis, a Orionis, ^ Tauri, and 5, fj., ^ Gemi- 

 norum, and " besides the parallehsms " he notes " a 

 most wonderful case of radiation of .stars and 'visps in a 

 fan-shaped group, 08 Orionis being approximately the 

 centre." He finds a preponderance of the groupings "at 

 an average delation of 15° from the direction of Gould's 

 Galactic Equator, viz. at a position angle of 345° with 

 that great circle, and more nearly parallel with a Galactic 

 Equator derived from Proctor's chart of the DunhiiiuKtenuuj 

 stars, and be adds : " One conclusion derived fi'om the in- 

 vestigation is that the stars and wisps in parallel lines are 

 probably in the same region of space ; and therefore that 

 the majority of the stars — at least of those down to the 

 9th or 10th magnitude — in extensive tracts of the area 

 examined are really near one another." 



An examination of Dr. Boeddicker's beautiful drawing 

 of the Milky Way seems to show that the Galaxy itself is 

 — at least, chiefly — composed of "star-streams" and 

 " star-sprays " and clustering groups of small stars, and 

 does not represent a " cloven " flat disc, as was originally 

 supposed. 



Mr. Proctor pointed out that " the nebular system also 

 shows the most marked tendency to stream-formation." 

 Li the great nebular region in Virgo and Coma Berenices, 

 he finds that " the stars are not arranged imiformly over 

 either region, but to some degree clusterLngly with inter- 

 spersed spaces relatively vacant. Now no nebula appear 

 in the more vacant spaces, nor do nebula appear chiefly 

 where the stars are more clustered. It is on the borders 

 of star-clusterings, and in the breaks of star-streams, that 

 the nebuhe show themselves, precisely as though they 

 had taken the place of stars where star-matter began to 

 fail." This fact, considered in connection with Laplace's 

 Nebular Hypothesis, is very remarkable and suggestive. 

 The nebulie seen in the streams may possibly represent 

 stars in their initial stage. 



WHAT IS A VOLCANO? 



By tue Rev. H. N. Hutchinson, B.A., F.G.S. 



IN old days volcanoes were regarded with superstitious 

 awe, and any investigation of their action would 

 have been considered rash and impious in the 

 highest degree. A certain "burning moimtaiu " 

 in the Lipari Isles, called Volcano, was considered 

 to be the forge, or workshop, of Vulcan, the god of Fire. 

 And so it comes about that all " burning mountains " take 

 their name from this island in the Mediterranean. 



In the present paper it will only be possible to consider 

 two aspects of the subject of volcanoes, which may, 

 perhaps be more suitably presented in the form of ques- 

 tions, viz. (1) WItat t.s a rokano .' (8) Wliat (or tht' chiif 

 phenomena of rolcanic action ! 



In the first place, a volcanic mountain consists of alter- 

 nating sheets of ash and lava, mantling over each other 

 in an irrogiUar way, and all sloping (or " dipping," as 

 geologists say) away from the centre. In the centre is a 

 pit, or chimney, widening out towards the top so as to 

 resemble a funnel or a cup. Hence the name " crater," 

 which means a cup. In the centre of the cone there is 

 fi-equently a little minor cone. As oiu' readers will pro- 



