18 THOUGHTS ON NATURAL PHILOSOPHY. 



in the same direction. It is also extremely remark- 

 able that all the great planets and many of the 

 smaller ones have their orbits very nearly in the 

 same plane, and nearly circular in form. Viewed 

 as a question in probabilities, we may ask what the 

 chance is that out of two hundred and fifty bodies 

 revolving around the sun all shall be moving in one 

 direction. If the direction of movement were merely 

 decided by chance, the probability against such an 

 arrangement is of stupendous magnitude. It is 

 represented by the ratio of unity to a number con- 

 taining about sixty figures, and so we are at once 

 forced to the conclusion that this remarkable feature 

 of the planetary motions must have some physical 

 explanation. In a minor degree this conclusion is 

 strengthened by observing the satellites. Discarding 

 those of Uranus, in which the orbits of the satellites 

 are highly inclined to the ecliptic, and in which 

 manifestly some exceptional though unknown in- 

 fluences have been at work, we may say that the satel- 

 lites revolve around the primaries also in the same 

 direction ; while, to make the picture complete, we 

 find that the planets, so far as they can be observed, 

 rotate on their axis in the same manner. 



The nebular theory here steps in and offers an 

 explanation of this most remarkable uniformity. 

 Laplace supposed that our sun had once a stupend- 

 ous nebulous atmosphere which extended so far out 

 as to fill all the space at present occupied by the 

 planets. This gigantic nebulous mass, of which the 

 sun was only the central and somewhat more con- 

 densed portion, is supposed to have a movement of 



