206 FROM NEBULA TO NEBULA 



great comet of 1843, which passed its perihelion within 

 33,000 miles (less than 1-25 of the sun's diameter) of the 

 solar surface, at a velocity of 342 miles a second, through 

 a hot blast capable of vaporizing steel, and in the teeth 

 of a coronal resistance of undoubted immensity yet 

 came through the ordeal apparently unscathed, and 

 without any perceptible retardation. More than this, it 

 brandished a tail 198 million miles in length! The tail 

 kept well behind the comet, consequently the tip of it 

 must have swept through space at the velocity of about 

 40,000 miles in one second. Supposing that the tail aver- 

 aged this length for as much as a single week, then, ac- 

 cording to the hypothesis of light repulsion, the comet 

 must have simultaneously grown and lost some 700 of 

 these appendages! 



Of Encke's comet we are told, first, that its period is 

 diminishing, second, that it is not doing so as fast as for- 

 merly, and, third, that its head gradually contracts as 

 it nears the sun and expands again as it recedes. As- 

 tronomers have been able to suggest only one way to ex- 

 plain the first two phenomena, namely, by postulating the 

 ether as slightly resistant "not enough to hurt, yet just 

 enough to serve". Newton, you remember, was scrupu- 

 lously particular to have it understood that the ether 

 is absolutely imponderable, knowing as he did full well 

 that even the least possible retardation of the planets 

 would inevitably destroy the " divinely-ordained " equal- 

 ity of the centripetal and centrifugal forces and bring the 

 whole system to grief. But even if we are gracious 

 enough to wink at this modern indiscretion, or to concede 

 the paradox of a medium by its resistance causing a 

 hastening of the comet, as these ingenious savants de- 

 clare, we cannot decently go further and admit that the 

 degree of that resistance is declining in the same gallop- 

 ing ratio as required by the premises. 



The mystery is easily cleared when it is recognized 

 that Encke's comet is a comparatively recent intruder 

 into the solar system; as is indicated by the fact of its 

 having the highest eccentricity among the short-period 



