When first noticed, Donati’s comet had, like all large comets, a bright envelope of light round the nucleus. After a while the one envelope grew into three envelopes, and a new tail formed beside the principal tail, which for a time was seen to bend gracefully into a curve, like a splendid plume. A third, but much fainter tail, also made its appearance, and many angry-looking jets were poured out from the nucleus. These changes took place while the comet was passing through the great heat of near neighborhood to the sun. Afterwards, as he passed away, he seemed gradually to cool down and grow quiet. The singular changes in the appearance of Newton’s comet have been earlier noticed. No marvel that he did undergo some alterations. The tremendous glare and burning heat which that comet had to endure in his rush past the sun, were more than twenty-five thousand times as much as the glare and heat of the fiercest tropical noonday ever known upon earth. Can we wonder that he should have shown “signs of great excitement,” that his head should have grown larger and his tail longer? It certainly was amazing, and past comprehension, that the said tail, over ninety millions of miles in length, should in four days have seemingly swept round in a tremendous half-circle, so as first to point in one direction, and then to point in just the opposite direction. We are much in the habit of speaking about comets as traveling through the heavens with their tails streaming behind them. But though this is sometimes the case, it is by no means always. The tails of comets always stream _away from the sun_--whether before or behind the comet’s head seeming to be a matter of indifference. As the comet comes hurrying along his orbit, with ever-increasing speed, towards the sun, the head journeys first, and the long tail follows after. But as the comet rounds the loop of his orbit near the sun--the point nearest of all being called his _perihelion_--the head always remains towards the sun, while the tail swings, or seems to swing, in a magnificent sweep round, pointing always in the direction just away from the sun. Then, as the comet journeys with slackening speed, on the other side of his orbit, towards the distant _aphelion_, or farthest point from the sun, he still keeps his head towards the sun. So, at this part of his passage, in place of the head going first and the tail following after, the tail goes first and the head follows after. The comet thus appears to be moving backwards; or, like an engine pushing instead of drawing a train, the head seems to be driving the tail before it. [Illustration: GREAT COMET OF 1744.] [Illustration: THE FIRST COMET OF 1888--JUNE 4.]