GIFT-BOOKS.
ASTRONOMY FOR CHILDREN.* Mn. JIIITTox has done, we do not doubt, all that is possible to carry out the purpose indicated in his title. But astronomy is a subject which tries the skill of an expositor to the utmost.
" Erratic " and "reflective power," to take examples at random, are not within every one's comprehension; nor could every "grown-up," even among educated people, at once answer the
question: "What is an ellipse?" The writer of this notice has just put the question to three who may be so described,—
one thought it something left out in a sentence; the second had no idea ; and the third knew. We .say this, not at all in the way of censure, but to give a fair warning. An elder who may give this volume as a present—and he or she can hardly do better—must be prepared for some puzzling inquiries. Still, when we say that Mr. /Slitton's book comes
with a virtual recommendation from the late Agnes Clerke, who had intended to write the preface, it will be seen that it is the nature of the subject, not any fault of handling, that makes the difficulty. A good illustration is supplied by the suggestion of the frontispiece, though the difficulty is not so much in seeing what should be done as in doing it. Two children are to represent earth and moon, while a "grown-up," who, as we are reminded, ought to be enormously big, stands for the sun. The moon-child has to go round the earth- child, always keeping her face towards her; the earth-child must spin like a top, and both together must move along a circular path. That is an effect which will not be successfully reached in one or two rehearsals. Where all is so interesting it is not easy to make a choice. Perhaps the chapter on Mars may furnish a good speci- men of the fascination which the subject exercises. The moon must be a great disappointment. To think that this beautiful object is an utter desolation! The reflection of what uses it serves does not go far to make up for it. But when we come to Mars there is a touch of quasi-human interest. What may not the canals mean! Their existence is now an acknowledged fact ; the camera has established it. The green and red patches which we used to think were sea and land are now with more probability supposed to be vegeta- tion and desert. The doubling of the canals may come from the growing up of borders of vegetation alongside of them ; vegetation induced by the water supplied from the melting of the polar ice-caps. All this gives a sense of reality, whereas when we get to Jupiter or Saturn what are we to think of worlds which are not heavier than, or, as is the case with Saturn, not so heavy as, water ? The moons of Mars touch the fancy in another way. There never was a more strange coincidence than that Dean Swift should invent as a climax of Laputa absurdities that their astronomers had discovered two moons attending Mars, and OA one of them went round the planet in about ten hours. There are two moons, and one of them goes round the planet in between seven and eight hours. The Jupiter moons have quite a different claim on our respect. It is from them we learnt the speed of light. They did not disappear and reappear at the time that was calculated for them,—and the reason was this. When Jupiter is on the far side of his orbit he is three hundred millions of miles further off than when he is on the near side. The further off he was the later were the phenomena of the moons, because the light bad the additional distance to travel.
The planets disposed of, we come to the sun. Here is one of the facts about it which can be easily stated. Why does it go on giving out heat and light? A few meteors fall into it, but they cannot count for much. It keeps on contracting— it was the contracting of the original nebula that made it what it is—and the process keeps up its heat. "It has been calculated that if the sun contracts two hundred and fifty feet in diameter in a year, the energy thus gained and turned into heat is quite sufficient to account for the whole yearly output." After the sun come in succession the comets, the meteors, and then, with fascinating resemblances to various earthly things, the stars. And here, of course, there are marvels without end. What could be more strange than the relation of Algol and the dark body which eclipses him every third day. Imagine a planet nearly as big as its sun going round it in three dap; I This is an excellent book on a most fascinating subject.
• The Childrett's Book of Stars. By G. E. Mitton. London : A. & C. Black. [ea]
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