268 POPULAR SCIENCE MONTHLY. 



(Herschel, 'Outlines of Astronomy/ p. 376.) 



"It is for the most part after thus passing the sun, that they shine forth 

 in all their splendor, and that their tails acquire their greatest length and 

 development; thus indicating plainly the action of the sun's rays as the excit- 

 ing cause of that extraordinary emanation." 



Again (p. 566). 



"The tail of the great comet of 1680 immediately after its perihelion pass- 

 age was found by Newton to have been no less than 20,000,000 leagues in 

 length, and to have occupied only two days in its emission from the comet's 

 body! a decisive proof this of its being darted forth by some active force, the 

 origin of which, to judge by the direction of the tail, must be sought in the sun 

 itself." 



Now a particle with one- half the critical diameter would in the 

 course of traveling from the sun's surface to a distance equal to his 

 radius acquire a speed of 430 kilometers per second. With this velocity 

 it would cross a space equal to the diameter of the sun, 865,000 miles, 

 in less than an hour. In comets' tails we probably have to do with 

 particles having less than one eighteenth of the critical diameter. Such 

 particles would cover the same distance in less than four minutes. With 

 a force many times the sun's attraction driving them into space, they 

 would make little of 20,000,000 leagues in two days; whereas if this 

 were to be accomplished against gravity the velocity of projection re- 

 quired might well stagger the astronomers. 



Eeferring to Halley's comet, Herschel says (p. 381) : 



On the 2d of October [the very day of the first observed commencement of 

 the tail) the nucleus, which had been faint and small, was observed suddenly 

 to have become much brighter, and to be in the act of throwing out a jet or 

 stream of light from its anterior part, or that turned toivards the sun. This 

 ejection after ceasing a while was resumed, and with much greater apparent 

 violence, on the 8th, and continued with occasional intermittences so long as 

 the tail itself continued visible. . . These jets, though very bright at their 

 point of emanation from the nucleus, faded rapidly away, and became diffused 

 as they expanded into the coma, at the same time curving backwards, as 

 streams of steam or smoke would do, if thrown out from narrow orifices, more 

 or Jess obliquely, in opposition to a powerful wind, against which they were 

 unable to make way, and ultimately yielding to its force, so as to be drifted 

 back and confounded in a vaporous train, following the general direction of the 

 current. 



It seems impossible to avoid the following conclusions. 1st. That the 

 matter of the nucleus of a comet is powerfully excited and dilated into a vapor- 

 ous state by the action of the sun's rays, escaping in streams and jets at those 

 . points of the surface which oppose the least resistance. 2ndly. That this 

 process chiefly takes place in that portion of the nucleus which is turned 

 towards the sun; the vapour escaping chiefly in that direction. 3rdly. That 

 when so emitted, it is prevented from proceeding in the direction originally 

 impressed on it, by some force directed from the sun, drifting it back and 

 carrying it out to vast distances behind the nucleus, forming the tail. 4tlily. 

 That this force, whatever its nature, acts unequally on the materials of the 



