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Page:Popular Science Monthly Volume 34.djvu/871

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vation than even the prominences ever reach, and seems to be not wholly gaseous, but to contain, besides the hydrogen and the mysterious 'coronium,' dust and fog of some sort, perhaps meteoric."

While the part of the book devoted to the sun is so excellent, it must not be supposed that the other divisions of the subject have not received adequate attention. In fact, one of the strong points of the work is that it is well balanced, and the student gets as complete a view of the science as is possible within the limited space of a school text book. The author, in setting forth with uncommon clearness the elementary mathematics of his subject, has not ignored what may be called its philosophical aspects. He has, for example, given an account of the nebular hypothesis which, notwithstanding its brevity, can not fail to be of much use in dissipating the fog that hangs over this whole subject in the minds of those who have little knowledge of astronomy beyond what has been vouchsafed to them in the ordinary college curriculum. We should have been pleased to see from Prof. Young's pen an elementary account of George Darwin 's remarkable theory of "tidal evolution," in place of the references that are given to other popular explanations of that subject.

Recent discoveries have carried us so far into the depths of space that there is nothing within the circuit of astronomical knowledge and investigation which appeals more strongly to inquiring minds than the relations of the solar system to the universe without. Such achievements in observation as those of the Lick telescope, and the recent surprising advances in astronomical photography, promise us much light upon the old problem of the structure of the heavens. Prof. Young's remarks in the present work on the distribution and motions of the stars, though brief, are fruitful in suggestion. We quote the following passage as a particularly interesting generalization: "In the solar system the central power is supreme, and perturbations or deviations from the path which the central power prescribes are small and transient. In the stellar system, on the other hand, the central force, if it exists at all (as an attraction toward the center of gravity of the whole mass of stars), is trifling. Perturbation prevails over regularity, and 'individualism' is the method of the greater system of the stars, as solar despotism is that of the smaller system of the planets."

This remark, which is fully justified by all we now know of stellar motion, presents a very different picture of the universe from that which has sometimes been drawn for the edification of admiring congregations, of planets circling around suns, and suns around other suns, and these systems around grander systems still, and finally the whole universe revolving with a stupendous orbital sweep around the great center of all, the throne of the Creator himself! It appears that things don't revolve that way.

There are many good features in the book that we should like to point out if space permitted. It may be remarked, by the way, that a fine example of the author's desire to convey practically useful information is the italicized sentence on page 35: "Never turn the hands of a chronometer backward."

It goes without saying that the more mathematical parts of Prof. Young's work are highly excellent, succinct, and clear. Such subjects as central forces, the tides, parallax, the equation of time, and perturbations are treated in such a way as to give the student a sure insight into the nature of the problems involved.

The illustrations accompanying the text are good, many being original, and some excellent ones borrowed from the author's book on "The Sun" (published by Appletons), and from other sources. Some useful tables of elements and constants, based upon the most recent information, close a book which, it is not too much to say, will be scarcely less welcome to the general reader than to the students for whose instruction it is intended.

Force and Energy: A Theory of Dynamics. By Grant Allen. London and New York: Longmans, Green & Co. Pp. 161. Price, $2.25.

Mr. Allen's theory does not aim to revolutionize the generally accepted ideas concerning force, but is rather an attempt to classify the known forms of force, and systematize their relations. Under the general term power he includes forces and energies, distinguishing them by their effects. Thus,