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 exciting cause of that extraordinary emanation."
Again (p. 566).
"The tail of the great comet of 1680 immediately after its perihelion passage 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 required might well stagger the astronomers.
Referring 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 towards 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 less 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 vaporous 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. 4thly. That this force, whatever its nature, acts unequally on the materials of the