round like a stick, but only like a jet of water from a hydrant. The curvature would, therefore, afford data for finding the velocity with which the repelled matter was driven off.
Again, the convex, or front side of the tail is brightest and straightest. In Donati's comet, 1858, small straight tails preceded the main tail.
One explanation serves for all these facts. The sun analyzes the matter of the comet. Some parts of it he can dart away at an incomparably higher velocity than others. This matter, most swiftly ejected, either makes separate straight tails, as in 1858, or somewhat straightens and brightens the convex, or front, side of the main tail.
There is much room for speculation and enquiry in connection with this repulsive force. Does it pursue the repelled matter, and drive it away with an ever-increasing motion, so that it will leave our system altogether, or does it give an initial impulse, and have done? Will the repelled matter change its condition by cooling, and cease to be liable to the persecution of the repelling force? Or has it lost its affinities and the power of changing its condition? Whatever may be its destiny it is certainly divorced from the comet for ever.
There is reason to suppose that all the matter of a comet is not susceptible to this repulsive force, and that a sufficient number of perihelion passages will sift all the susceptible matter out, and leave the comet incapable of producing a tail. Hence almost all the short-period comets that are in perihelio every few years are tailless; while visitors from the eternities of space, who can only be in that sifting position once in ten or twenty millions of years, frequently make a prodigious display of tail.
Art. LXI.—Macquarie Island. By John H. Scott, M.D., F.R.S.E.,
Professor of Anatomy and Physiology in the University of Otago.
[Read before the Otago Institute, 21st June, 1881 and 9th May, 1882.]
Plate XXXIX.
In most of the maps which I have seen, an island named "Emerald" is put down in latitude 57°, a long way to the south of Macquarie Island. This, is, however, now generally regarded as mythical, for its supposed site was sailed over by the American Transit of Venus Expedition and no land was observed. In all probability its discoverers mistook an iceberg for snowcovered land, a not unlikely mistake in misty weather.
We may therefore safely consider that Macquarie Island is the most southerly island of the outlying members of the New Zealand group, indeed,
