Unless there does exist a trans-Neptunian planet, a Neptunian astronomer will know very little about planets, for Uranus and Saturn will alone be visible to him. Both will of course be what we call “inferior planets,” and under the best of circumstances will cut a poor figure in the Neptunian sky. CHAPTER XIII. COMETS. I suppose that it is the experience of all those who happen to be in any sense, however humble, specialists in a certain branch of science, that from time to time, they are beset with questions on the part of their friends respecting those particular matters which it is known that they have specially studied. There is no fault to be found with this thirst for information, always supposing that it is kept within due bounds; but my motive for alluding to it here, is to see whether any well-marked conclusion can be drawn from it, within my own knowledge as regards astronomical facts or events. Now in the case of the science of astronomy (for which in this connection I, for the moment, will venture to speak), there is certainly no one department which so unfailingly, at all times and in all places, seems to evoke such popular sympathy and interest as the department which deals with Comets. Sun-spots may come and go; bright planets may shine more brightly; the Sun or Moon may be obscured by eclipses; temporary stars may burst forth,—all these things are within the ken of the general public by means of newspapers or almanacs, but it is a comet which evokes more questionings and conversations than all the other matters just referred to put together. When a new and bright comet appears, or even when any comet not very bright gets talked about, the old question is still fresh and verdant—“Is there any danger to the Earth to be apprehended from collision with a Comet?” followed by “What is a Comet?” “What is it made of?” “Has it ever appeared before?” “Will it come back again?” and so on. Questions in this strain have more often than I can tell of been put to me. They seem the stock questions of all who will condescend to replace for five minutes in the day the newest novel or the pending parliamentary election. It may be taken as a fact (though in no proper sense a rule) that a bright and conspicuous comet comes about once in 10 years, and a very remarkable comet every 30 years. Thus we have had during the present century bright comets in 1811, 1825, 1835, 1843, 1858, 1861, 1874 and 1882, whereof those of 1811, 1843, and 1858 were specially celebrated. Tested then by either standard of words “bright and conspicuous,” or “specially celebrated,” it may be affirmed that a good comet is now due, so let us prepare for it by getting up the subject in advance.