SEPTE^^^iER 2, 1895 



KNOWLEDGE. 



207 



ment, at any rate, they are still plastic. Their ultimate 

 configurations belong to the future. Even Jupiter's tiny 

 " fifth satellite " is forging its way outward under compul- 

 sion of the perennial ripple due to its difiereutial attraction 

 on the huge globe it circuits ; although at so tardy a rate, 

 that our author inclines to connect its interior position with 

 its small mass. The feebleness of its tide-raising power 

 obliged it to remain behind its companions, and indirectly 

 occasioned the notable eccentricity of its orbit by subjecting 

 it at close quarters to Jovian tidal friction. There is no 

 sign of its l)eing more juvenile than the Galilean quartet. 



The recession of a satellite from its primary implies, 

 as has been said, a transference of momentum. It implies 

 also, as a condition precedent to its occurrence, a period 

 of rotation for the primary shorter than the period of 

 revolution of the satellite. Should this relation be inverted, 

 the direction of the transference of momentum will also be 

 inverted. The attendant body will move along gradually 

 narrowing spires ; while, under the inverted influence of a 

 lay;ii)i<i tide-wave, the rotation of the dominant body will 

 be correspondingly accelerated. A destructive collision 

 must be the normal result. The inner satellite of Mars is 

 indeed thus circumstanced, yet may continue to subsist 

 during incalculable ages. But, in the first place, it is of 

 negligeable mass ; the height of the tidal wave raised by 

 it under the most favourable circumstances should be 

 measured by millimetres. In the second. Mars is probably 

 in no state to yield a sensible tide, even under a far greater 

 stress than Phobos is capable of producing. 



We can now easily see that the history of satellites must, 

 in general, be a blank beyond that "critical" past epoch 

 when they revolved synchronously with their primaries' 

 rotation. For, if they fell behind by a hair's breadth, 

 their doom was sealed ; they were eventually swallowed up. 

 Of these abortive satellites, Phobos alone, owing to his 

 puny stature, escaped destruction. 



The circumstances under which that critical epoch 

 occurred give the only available clue to the modes of 

 satelhte-production. The comparatively large size of our 

 moon, and the great power of the friction -bralie applied 

 by it to terrestrial rotation, are convincing proofs that the 

 antique earth spun vastly quicker than the modern earth — 

 its speed being such that the moon, when it swung round 

 at the same rate, must have travelled in an orbit scarcely 

 more than eight thousand miles across. Hence the 

 inference that the pair of globes flew asunder when nearly 

 of their actual density. 



But their case is exceptional. Jupiter and Saturn can 

 have surrendered but little of their rotational momentum 

 through tidal friction, although that little constituted a 

 considerable addition to the orbital momentum of their 

 comparatively insignificant satellites. Hence none of those 

 satellites can, even at the very beginning of their separate 

 existence, have approached within less than sixty to eighty 

 thousand miles of the actual planetary surfaces. Their 

 origin thus remains cloudy. Mr. Nolan is persuaded that 

 they sprang from primeval formations on the model of 

 Saturn's rings. Yet it appears gratuitous to postulate the 

 former existence of sets of appendages no traces of which 

 have survived. Uranus and Neptune especially, since they 

 have e\-idently made less progress in development than 

 Saturn, would surely have kept, if they ever possessed 

 such formations. Nature rejects our ideas of imiformity. 

 Creative power has many resources, and the lesson of 

 variety is enforced by the contrast between the biographies 

 of the moon and her compeers. The moon is unique in the 

 solar system — unique in her oi-igm, unique (not improbably) 

 in her solitary condition. The two peculiarities are 

 certainly not unconnected. 



PHOTOGRAPHS OF ELLIPTICAL' AND SPIRAL 



NEBULAE. 



By IsAic EoBEKTs, D.Sc, F.R.S. 



SPIRAL NEBULA M. 74 PISCIUM. 

 R.A. lb. 31m. 19s., Decl. North 15° 16'. 

 Scale, 1 millimetre to 12 seconds of arc. 



THE photograph was taken with the 20-inch reflector 

 on December 9th, 1893, between sidereal time 

 Ih. 7m. and 5h. 21m., with an exposure of the 

 plate during three hours and forty minutes. 



References. 



N. G. C. No. 62S, G. C. No. 372, h 142. 



Sir J. Herschel (G. C. 372, p. 52) states, as the result of 

 eleven observations, that it is a globular cluster, faint, very 

 large, round, pretty suddenly much brighter in the middle, 

 partially resolved. 



Lord Rosse {Phil. Trans. 1861, p. 711) designates it as 

 a spiral nebula with the centre formed of stars, and several 

 stars visible through the nebula. A marginal sketch is 

 given which shows the spiral form, and in the Obsen-ations 

 of Xi'hula iind Clusters of Stars, p. 21, another marginal 

 sketch is given which shows a nucleus and five stars 

 involved in the spirals. 



The photograph shows the nebula to be a very perfect 

 spiral with a central, nebulous, stellar nucleus and a 15th 

 magnitude star close to it on the south side. The convo- 

 lutions of the spiral are studded with many stars and 

 star-like condensations, and on the north pn'Cidiwj side 

 there is a partial inversion of one of the convolutions 

 which is suggestive of some irregular disturbing cause 

 having interfered with the regular formation of a part of 

 that convolution. 



ELLIPTICAL NEBULA Ijl . I. 84 COM^ 

 BERENICIS. 

 R.A. 12h. 45m. 333., Decl. North 26° 3'. 

 Scale, 1 millimetre to 12 seconds of arc. 



The photograph was taken with the 20-inch reflector 

 on May 7th, 1894, between sidereal time 12h. 31m. and 

 14h. Im., with an exposure of the plate during ninety 

 minutes. 



References. 



N. G. C. No. 4725. G. C. No. 3249, h 1451. 



Sir J. Herschel {G. C. 3249) describes the nebula as 

 very bright, very large, extended, very gradually then very 

 suddenly very much brighter in the middle, with an 

 extremely bright nucleus. 



Lord Rosse (Obs. of Xeli. and CI. of Stars, p. 121) records 

 six observations of the nebula made between 1850 and 

 1867, and describes it as another spiral with two arms 

 and some stars in the foUowbvi arm ; very large and very 

 bright. The centre itself is like an elongated nebula with 

 a nucleus, and enveloped in an irregular ring or rings of 

 nebulous light. He gives a rough marginal sketch of it. 



The photograph shows the nebula to be a symmetrical 

 ellipse, and not a spiral, with the major axis in narth 

 foUoirhhi to Sdiith precedinij direction, and the nucleus to 

 be a nebulous star of about the 12th magnitude. Both 

 the star and the nebulosity surrounding it have well- 

 defined margins, the nebulosity having a ring-lilie 

 boundary. Surrounding the nucleus, at a great distance, 

 is a well-defined ring, but deformed on the nurth fnUoiriny 

 side, and in the ring are involved several star-hke con- 

 densations of nebulosity. Outside this ring is another 

 faint ring, symmetrical with it, and also discontinuous, 

 like the first, on the north follou-inij side ; and there is in 

 it some evidence of the existence of another very faint 

 ring outside this. In the divisions between the rings are 



