212 THE BORDERLAND OF SCIENCE. 



with him, and they would appear as comets having a 

 direct, not a retrograde, motion.* And in like manner 

 matter ejected even at a considerable inclination to the 

 level in which Jupiter travels would yet travel nearly 

 in that level, just as a ball which a passenger by an 

 express train should attempt to fling straight upwards 

 would in reality not travel vertically upwards but 

 slantwise with respect to the ground. 



It certainly seems to be a circumstance strongly 

 favouring the new theory that this relation precisely 

 accords with the observed peculiarities of the Jovian 

 family of comets. It had been noticed respecting them 

 (long before this theory had been thought of) that they 

 all advance, and that they all travel on paths mode- 

 rately inclined to the general level of the planetary 

 motions. Sir John Herschel spoke thus in a lecture 

 delivered in 1859, and published among his Familiar 

 Essays : c It is a very remarkable feature that all 

 the comets of short period revolve in the same direction 

 round the sun as the planets, and have their orbits in- 

 clined at no very large angles to the ecliptic.' 



Now if we turn to Neptune, which travels at the 

 rate of only three miles per second (while Jupiter 

 travels eight miles per second), we should expect to 

 find a different state of things. We might still expect, 



* The case may be compared to that of water flung (not too sharply) 

 over the stern of a swiftly advancing ship ; such water would move 

 "backwards with respect to the ship, but seen from some station at rest 

 would seem to move forwards. As Jupiter travels at the rate of eight 

 miles per second, it is readily seen that matter retiring very swiftly 

 from him rearwards would yet be advancing in reality. 



