Yet here, where we seem to have our first firm ground for hypothesis respecting these mysterious objects—comets’ tails—we meet with stupendous difficulties. Consider, for instance, the phenomena presented by Newton’s comet. That comet had traversed the last ninety millions of miles of its approach toward the sun in four weeks. At the end of that time it passed out of view for a few days, having then a tail ninety millions of miles, at least, in length. Four days passed, and it reappeared on the other side of the sun—having in the interval traversed nearly a semi-circle—in reality, of course, the perihelion end of its long oval path. At its reappearance, it had a tail still ninety millions of miles in length, but the tail with which it reappeared had, of course, a direction entirely different from that of the tail which had been seen before—the two directions were inclined about one hundred and sixty degrees to each other. Now, as Sir John Herschel remarks, we can not look on the tail of a comet as something whirled round like a stick, as the comet circles round its perihelion sweep. The tail with which the comet reappeared must have been an entirely new formation. Nor can we doubt that if the comet had been watched as it swept around the sun, the changes in the tail’s position which had been observed to the time of disappearance, would have been observed to progress continuously, the tail passing by a uniform motion from the position it then had to that which it was observed to have at the time of reappearance. So that we may fairly suppose the tail with which the comet reappeared to have been formed in much less than the time during which the comet had been out of sight. Probably its farthest part had been formed in much less than a day, the part near the head being, of course, formed later. But if the matter repelled from the head was thus driven over a distance of ninety million miles in twenty-four hours, at the outside, the average velocity of its motion was about a thousand miles per second, or nearly three times as great as the greatest velocity which the sun _can_ communicate by his attractive energy to matter approaching him from without, even though such matter come to him from an almost infinite distance, and in a perfectly straight line—the conditions most favorable for giving a high rate of final velocity. Such velocity as the sun can thus give by his attractive energy is only given to matter which has been exposed a long time to his influence; but here, in the tail of the great comet of 1680, matter seems to have acquired almost instantaneously a velocity sufficing to carry it over ninety million miles with an average speed three times as great as the sun can thus, after long effort, communicate by means of his attractive power! The difficulty is so great that many efforts—some bold and daring, others positively wild in the unscientific absurdity of their nature—have been made to overcome it.