f its orbit, and even then it is
not a very well-defined point which admits of being measured with the
precision attainable in observations of a star or a planet. This comet
of 1843 is, however, especially remarkable for the rapidity with which
it moved, and for the close approach which it made to the sun. The heat
to which it was exposed during its passage around the sun must have
been enormously greater than the heat which can be raised in our
mightiest furnaces. If the materials had been agate or cornelian, or the
most infusible substances known on the earth, they would have been fused
and driven into vapour by the intensity of the sun's rays.

The great comet of 1858 was one of the celestial spectacles of modern
times. It was first observed on June 2nd of that year by Donati, whose
name the comet has subsequently borne; it was then merely a faint
nebulous spot, and for about three months it pursued its way across the
heavens without giving any indications of the splendour which it was so
soon to attain. The comet had hardly become visible to the unaided eye
at the end of August, and was then furnished with only a very small
tail, but as it gradually drew nearer and nearer to the sun in
September, it soon became invested with splendour. A tail of majestic
proportions was quickly developed, and by the middle of October, when
the maximum brightness was attained, its length extended over an arc of
forty degrees. The beauty and interest of this comet were greatly
enhanced by its favourable position in the sky at a season when the
nights were sufficiently dark.

On the 22nd May, 1881, Mr. Tebbutt, of Windsor, in New South Wales,
discovered a comet which speedily developed into one of the most
interesting celestial objects seen by this generation. About the 22nd of
June it became visible from these latitudes in the northern sky at
midnight. Gradually it ascended higher and higher until it passed around
the pole. The nucleus of the comet was as bright as a star of the first
magnitude, and its tail was about 20 deg. long. On the 2nd of September it
ceased to be visible to the unaided eye, but remained visible in
telescopes until the following February. This was the first comet which
was successfully photographed, and it may be remarked that comets
possess very little actinic power. It has been estimated that moonlight
possesses an intensity 300,000 times greater than that of a comet where
the purposes of photography are con