Sept., 1904.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



113 



met with only in the sidereal heavens, but are there 

 widely prevalent. Thoiiyh remote from our experi- 

 ence, they are fundamental in the realms of space. If 

 we could define and comprehend them we should he in 

 a better position for determinin"^ the cosmical status of 

 nebulje. 



The choice is open between two rival theories of 

 nebulous spirals. The first is the more obvious, and 

 readily falls in with admitted mechanical princijiles. 

 Sir Robert Ball has adopted and ingeniously adxocated 

 this view. 



A globular collection of promiscuously revolving 

 particles inclines, if left to itself, to flatten down into 

 a disc. The reason is this : In a system of the kind, 

 moment of momentum is invariable, while energy con- 

 stantly diminishes. To render the contrast intelligible 

 we have only to consider that moment of momentum is 

 the algebraic sum of all the products of mass and 

 motion in the aggregation, reduced to, or projected 

 upon its " principal plane," while energy is independent 

 of the varied directions of velocity. Collisions con- 

 sequently involve no diminution of moment of 

 momentum, but combine with radiative waste to pro- 

 duce a steady loss of energy. Inevitably, then, the 

 system will assume the form in which it possesses the 

 minimum of energy that is consistent with the main- 

 tenance of its original momentum ; and it is that of a 

 disc extended in the principal plane. Retrograde 

 movements will by this time have become eliminated ; 

 the constituent particles circulate unanimously in one 

 direction ; and Sir Robert Ball adds that their circula- 

 tion, owing to the more rapid rotation of the central 

 mass, is along spiral paths. * They would accordingly 

 present the twisted conformation so commonly ob- 

 served in the heavens, and might even include sub- 

 ordinate centres of attraction, fitted to ripen and 

 strengthen into a full-blown retinue of planets. .Such 

 are spiral nebula? regarded in their direct mcchanic.-il 

 aspect. .Spherical ncbuhe are their immediate progeni- 

 tors ; suns, with or without trains of dependent worlds, 

 their lineal descendants. 



Let us, however, consult some autographic records 

 and weigh attentively what these peculiar objects tell 

 us about themselves. We see at once that their curving 

 lines are not laid dow n at hap-hazard, but according to 

 a strictly defined plan. Spiral nebuhc are not formed 

 like watch-springs by the windings of a single thread. 

 They are always two-branched. From opposite ex- 

 tremities of an elongated nucleus issue a pair of 

 nebulous arms, which enfold it in double convolutions. 

 Their apparent superposition and interlacements oc- 

 casion, in the Lyra nebula, the noted effect of a fringed 

 and ruptured annulus, and it is of profound interest to 

 perceive that even in gaseous masses the same cf)M- 

 -Structive rule prevails as in the great Whirlpool in 

 Canes Venatici. 



It is, however, almost irreconcilable with the 

 hypothesis that an influx of material is in progress. 

 Falls due to gravity could not be limited to two narrow 

 areas on the central body. Matter ejected from it 

 might, on the other hand, quite conceivably follow this 

 course. Interior strain could easily be supposed to 

 cause yielding along a given diameter, and nowhere 

 else. Solar disturb;inces partially and dimly illustrate 

 such a mode of action. Diametrically opposite 

 prominences are not unknown. They indicate the 

 action of an explosive force right across the solar 

 globe. Similarly, the formation of a spiral nebula can- 



* The Earth's Beginnings, pp. '243-7. 



not be rightly apprehended otherwise than as the out- 

 come of long-conlinued, oppositelv directed eruptions. 

 The history of the hea\ens imolves the law ol 

 spirality. The scope ol its (ioniiiiion conliiuialh' widens 

 .is research becomes intensified. 1 lu' lluygenian 

 " portent " in the Sworil of Orion now figures as 

 merely the nucleus of the " grcnt winding Nebula" 

 photographed by Professor W. II. I'ii kering in 1889. 

 That the vast nebulosity encompassing the I'lei.ules is 

 an analogous structure seems eminently probalilc, 

 Ihiiugh the brilli.ancy of the enclosed stellar group 

 obliterates most traces ol its ground-plan. 'i'he 

 magnitude of the phenomenon, we are told by Professor 

 Harnard ' who (k^tected it in i<Scj3 by means of :i ten- 

 hours' exposure with the VV'illard lens, transcends our 

 [)owcrs of realisation. It covers 100 sc|uare degrees of 

 the sky with intricate details. About four minutes of 

 arc to the north-west of the King in Lyra lies .-i small 

 nebul.a discovered visually by Professor Karnaid in 

 i8()3, and photographically resohed by Keelcr into .1 

 delicate spiral. It is a two-branched, Irll-liaiuKd 

 spiral, as the large adjacent object has .also pioved to 

 be. One is, in fact, the miniature of the other, and ihey 

 are now shown, bv Professor .Schaeberle's short-focus 

 reflector, to be linked together by winding folds of 

 nebulosity into a compound spirrd system. The Dumb- 

 bell is held, on the same authority, to be similarly con- 

 ditioned, and the analftgv fre((uently noli'd in the 

 .aspects of these remarkable formations has thus be- 

 come incalculably widened in scale. 



The galactic relations of the Magellanic Clouds are 

 not easily defined. They are within the Milky Way, 

 yet not of it. Enigmatical excrescences upon the uni- 

 verse, they suggest an origin from gigantic eddies in 

 the onflowing current of sidereal arrangement. Their 

 miscellaneous contents are, at any rate, disposed along 

 eddying lines. Mr. H. C. Russell's photographs I 

 rendered t.his, in 1890, to some extent m.anifest, and 

 their indications were ratified by the Arequipa plates 

 from the study of which Professor flickering gained 

 the conviction that the great Looped Nebul.i, 30 

 Doradus, is the structm-al nucleus of the Nubecida 

 Major. " It seems," he wrote, 1 " to b"e the centre of 

 a great spiral, and to bear the relation to the entire 

 system that the nebula in Orion bears to the great 

 spir.al nebula which covers a Large p;ut of lli;if con- 

 slell.ation." 



On all sides, in the sidereal heavens, v\e can discern 

 the signs of the working of a law of convolution. 

 Sometimes they are patent to view ; sometimes half- 

 submerged ; but they can generally, with attention, be 

 disentangled from overlaying ajjpearani'es. They are 

 exhibited by stars no less than by nebida>, as the late 

 Dr. Roberts pointed out from convincing i)hotogra])hic 

 evidence ; the " hairy " ai)pen(lages of globular 

 clusters betray them by their curvilinear forms ; they 

 miH-t us in every corner of the wide nebular re:ilm'. 

 .M.-iny investigators recognise in the Milky Way itself 

 the stamp of spirality. Stephen Alexander, of Xew 

 jersey, S regarded the majestic galactic arch as a four- 

 branched .spiral, resulting from catastrophic breaches 

 in a primitive, equatorially loaded spheroid, thesti-eams 

 of matter eject(-d by which shoidd, owing to their lower 

 angular rotation, lag behind as they retreated from 

 the nucleus, and thus flow along helicoidal lines. 

 R. .'\. Proctor subsequently devised convoluted 





'Monthh Notices. Vol. LX., p. 259. 

 1 Sec K,wideJi;e. Vol. XIV., p. 50, 

 ; Ihuvard Annals, Vol. XXVI, p. 206. 

 J Astr.Jour., Vol, H., p, 100, 1852. 



