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These lectures, delivered at the Royal Institution at Christmas 1913, as the eighty-eighth course of Juvenile Lectures, were taken down at the time by a shorthand writer. When I entered on the revision for publication, my first intention was to abandon the language of the lecture-room, substituting a narrative form. But I found the translation attended by all sorts of difficulties, so that the task assumed somewhat alarming dimensions. At this point I happened to look again at Faraday's Chemical History of a Candle, and saw that he had not thought it necessary to depart from the lecturing mode: with great relief I therefore ventured to follow him in this matter.

There are passages where I will ask the indulgent reader to remember that many of my audience were of very tender years: there are others where he will perhaps kindly allow his thoughts to dwell on the parents who came with them. If the alternation of view-point is somewhat erratic at times, I trust he will make some allowance for the difficulties.

Very few alterations of importance have been made, although the two years which have slipped away between the giving of the lectures and the passing of the final proof-sheets have added their due share to astronomical history. It seemed desirable to note the discovery of a ninth Satellite to