Page:The story of the comets.djvu/187

X. envelopes moved upward at the general rate of something like 30 miles an hour. The first one rose to a height of about 18,000 miles, when it wasted away; but none of the others reached so far, disappearing at elevations lower and lower, the last being lost sight of at an elevation of about 6000 miles.

It has been calculated that at perihelion Donati's Comet travels at a speed of 30 miles a second; but that at aphelion its speed is only 234 yards a second.

Few comets excited greater sensation by their sudden appearance above the horizon than the great Comet of 1861 (No. ii. of that year). It was discovered by J. Tebbutt, an amateur astronomer, at Windsor, N.S.W., on May 13, previous to its perihelion passage, which took place on June 11. Passing from the Southern Hemisphere into the Northern, it became visible in this country on June 29, though it was not generally seen until the following evening. It is so rare for the inhabitants of the British Islands to have a big comet all their own, as it were, that in this case the multitude of observers and observations was so great that selection is difficult.

A good all-round description was that given by Sir John Herschel, who observed the comet at his house, "Collingwood," Hawkhurst, Kent. He says:—

"The comet, which was first noticed here on Saturday night, June 29, by a resident in the village of Hawkhurst (who informs me that his attention was drawn to it by its being taken by some of his family for the Moon rising), became conspicuously visible on the 30th, when I first observed it. It then far exceeded in. brightness any comet I have before observed, those of 1811 and the recent splendid one of 1858 not excepted. Its total light certainly far surpassed that of any fixed star or planet, except perhaps Venus at its maximum. The tail extended from its then position, about 8° or 10° above the horizon, to within 10° or 12° of the Pole-star, and was therefore about 30° in length. Its greatest breadth, which diminished rapidly in receding from the head, might be about 5°. Viewed through a good achromatic, by Peter Dollond, of 2 inches aperture and 4-feet focal length, it exhibited a very condensed central light, which might fairly be called a nucleus; but, in its then low situation, no other physical peculiarities could be observed. On the 1st instant it was seen early in the evening,