270 HISTORY OF SCIENCE. 



rotation, would separate last, and would possess the greater velocities. 

 There would thus be produced a series of rings revolving in nearly 

 the same plane, and possessing greater velocities as they were nearer 

 the central mass ; but these rings would not continue to exist as such 

 unless they possessed a perfectly uniform structure throughout. Each 

 ring would therefore break up into several masses, which would assume 

 spheroidal forms in consequence of their fluidity, and would also con- 

 tinue to rotate in the same direction as the original rings. Each of 

 these spheroidal masses would also have a rotation about its own 

 axis, and would, with the vaporous atmosphere around it, repeat on a 

 smaller scale the same series of phenomena as the original nebular 

 mass; hence the origin of rings like that of Saturn and of satellites such 

 as belong to the earth, Jupiter, Saturn, etc. The nebular hypothesis 

 explains the central sun ; the spheroidal planets revolving at various 

 distances and with various velocities round it ; the satellites which at- 

 tend the larger planets; the ring of Saturn; the revolutions of planets 

 and satellites about their own axes, and the fact that all these revolu- 

 tions take place in one direction. Comets are explained in Laplace's 

 cosmogony as nebulous masses not originally any part of the great 

 nebula from which our system was consolidated, but smaller wandering 

 nebulse which the attraction of the sun has caused to deviate towards 

 him, and in some cases has compelled to travel in very elongated orbits 

 about him. This well accords with the fact of the orbits of comets 

 lying in planes inclined at all angles with that of the sun's equator, in 

 striking contrast with the comparatively small inclination of the pla- 

 netary orbits to that plane. Such wandering masses must also have 

 sometimes been actually drawn into the body of the sun, and the per- 

 turbations produced by their action, when passing through the inter- 

 planetary spaces at the period when the condensation was in progress, 

 .must have caused the orbits of the planets to deviate from their origi- 

 nal plane, which would otherwise have shown an exact coincidence 

 with that of the sun's equator. 



Soon after sunset, especially in the months of February and March, 

 a peculiar nebulous light may be observed in the western sky, extending 

 upwarjis from the place where the sun has set. It has a conical form, 

 or rather the figure which would result from a lens-shaped luminous 

 atmosphere surrounding the sun, as represented in Fig. 127. It is seen 

 extending 50* at least from the sun, and its breadth at the base is be- 

 tween 8 and 30, but it is so ill denned at the edges that its limits 

 cannot be certainly denned. It is seen to great advantage in the tropics, 

 but in this country it rarely appears equal in brilliance to the Milky 

 Way ; occasionally, however, it has been so bright that some persons 

 have mistaken it for a comet. Whatever may be the cause of this re- 

 markable phenomenon, it is assuredly connected witli the sun, for its 

 axis is always exactly in the plane of the sun's equator, and as this 

 is not very different from that of the zodiac, the luminosity is known 



