266 Mr. J. Ennis on the Physical and Mathematical 



an excess of *366. Thus it appears that the force of gravity 

 is enough, and a little more than enough, when acting on a 

 nebulous mass, to cause a rotation with sufficient velocity to 

 make the centrifugal equal to the centripetal force, and there- 

 fore to separate nebulous rings from the equatorial zone. — 

 Q. E. D. 



But what became of the excess (*366) of the velocity due to 

 the force of gravity ? Plainly that proportion of the force was 

 expended by the friction of the rotating exterior layer on the 

 unrotating interior. As I have already shown, the rotation 

 of some of the ancient nebulae began on the exterior ; and there- 

 fore the exterior layer must necessarily have been retarded by 

 friction on the interior layers. By calculation I have found that 

 this retardation of the velocity due to gravity, all the way from 

 Neptune to Uranus, amounted to 1 per cent, in a radial dis- 

 tance of contraction of 125,000,000 miles ; that is, down an 

 inclined plane 125,000,000 miles in height (not in length) the 

 velocity due to gravity was retarded 1 per cent. This retar- 

 dation became more and more as the unrotating centre of the 

 nebula was approached. The following Table gives the num- 

 ber of miles, not in diameter, but in radial contraction, from 

 planet to planet, necessary to cause a retardation of 1 per cent, 

 in the velocity due to the force of gravity as the sole moving 

 power in the case. 



From Neptune to Uranus 125,000,000 miles. 



„ Uranus to Saturn 80,000,000 „ 



„ Saturn to Jupiter 38,000,000 „ 



„ Jupiter to Mars 13,000,000 „ 



„ Mars to Earth 5,783,000 „ 



„ Earth to Venus 4,554,000 „ 



„ Venus to Mercury 4,459,000 „ 



This retardation was very small ; and the figures show a 

 wonderfully close agreement between the theory and the facts. 

 These facts are vastly multiplied when we take the asteroids 

 into account, as I have done. 



As we might anticipate a priori, this retardation becomes 

 much larger toward the unrotating centre of the solar nebula. 

 Interior to Mercury it was enormous. At the orbit of Mer- 

 cury the equatorial surface of the solar nebula rotated with a 

 velocity of 110,000 miles per hour. But it became so greatly 

 retarded by friction on the dense unrotating core, that now 

 the rotation of the solar equator is only 4500 miles per hour. 

 This explains why no planetary ring could be abandoned by 

 centrifugal force interior to Mercury. It also explains why 

 the equatorial region of the sun now makes a rotation in two 



