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.^[1]

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