ch. v.] PLANETARY EVOLUTION. 3G3 



the solar nebula. M. Plateau's experiment consists in freeing 

 a fluid mass from the action of terrestrial gravity, so that its 

 various parts may be subject only to their own mutual 

 attractions ; and then in imparting to this mass an increas- 

 ingly rapid movement of rotation. A quantity of oil is 

 poured into a glass vessel containing a mixture of water and 

 alcohol, of which the lower strata are heavier than the oil, 

 while the upper strata are lighter. The oil, when poured in, 

 descends until it reaches the stratum of the same density 

 with itself, when being freed from the action of terrestrial 

 gravity, and subjected only to the mutual attraction of its 

 own molecules, it assumes a spherical form. By an ingenious 

 mechanical contrivance, M. Plateau now causes the sphere of 

 oil to rotate about its own centre of gravity. While the 

 movement is slow, the excess of centrifugal force at the 

 equator of the oil-globe causes a bulging of the equator and 

 corresponding flattening of the poles, like that observed 

 in the sun and in all the planets. Prom a sphere the oil- 

 globe becomes a " spheroid of rotation." If now the move- 

 ment is considerably accelerated the equatorial portion of 

 the oil-globe becomes detached, and surrounds the central 

 sphere of oil in the shape of a nearly circular ring, like 

 Saturn's ring-system. Finally, if the movement is kept 

 up for a sufficient length of time, the oil-ring breaks into 

 fragments, which revolve like satellites about the oil- 

 globe, and each of which keeps up for a time its own move- 

 ment of rotation in the same direction with the revolution of 

 the ring. 



The common origin of the planets from the sun's equator, 

 as thus strikingly illustrated, explains at once the otherwiso 

 inexplicable coincidence of their rotations, their revolutions, 

 and their orbital planes. At a single glance we see why the 

 planetary orbits are always nearly concentric and nearly in a 

 plane with the solar equator ; and we see that, since the sun 

 must always have rotated, as at present, from west to east, 



