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 Jupiter, by Mr. S. B. Nicholson, in the table on p. 180 and on p. 181; but footnotes have been used for this addition, in order to keep the text nearer the original date. Possibly I have overlooked something in dealing with similar recent discoveries: Astronomy moves fast in these days, and it is not easy to keep the pace she sets. The hypothesis of a Sunspot-swarm of meteors, given on pp. 200-206, has been added intentionally. At the time of the lectures it had only just been formulated, and although the picture on p. 205 was shown, little was said on the matter. Two years' consideration has strengthened my confidence in this interpretation of the facts known to us, without producing any objections of a fatal character; and it seemed to me therefore that, as I was writing a book, I ought to put it in; to omit it might be interpreted as a lack of confidence on my part. At the same time I do not wish to ignore the attitude of other astronomers, which is duly acknowledged on p. 205. I have made every effort to acknowledge the source of the illustrations and to obtain permission for their publication where needful. But there may be some oversights. If so, I would beg the same kind consideration for the lecturer turned author as is extended to him from all quarters during his preparation of the lectures, immediately he mentions the magic words "Royal Institution" and "Children's Lectures."