Page:Popular Science Monthly Volume 54.djvu/728

706 currents, air movements, depth of water, etc., render these theories practically valueless for use in tidal calculations.

In the following section Dr. Draper shows how, by means of Lord Kelvin's "harmonic analysis," which separates the tide-generating forces of each kind into a number of ideal components, results of practical value are obtained. In Chapter XIII a very ingenious instrument for tide prediction which has been in use for some time by the Indian Government is described. The recording part of the machine is simply a paper-wound drum, on which a pencil point makes a graphic record. When the tides of a given port are desired, it is only necessary to set the instrument according to the tidal components, obtained by harmonic analysis and the time chosen for the beginning of the tide table, and then start it at the proper moment. It takes about four hours to run off the tidal curve for a year. This curve is then measured, and the year's tide table readily made out. Dr. Darwin informs us that a very similar instrument is now in course of construction for the United States Government. The remainder of the work consists of a more detailed discussion of the various disturbing influences which interfere with the simplicity of tidal movements—displacement of the earth's axis, earthquakes, etc, a long discussion of tidal friction, a study of the laws of rotating liquid masses, the nebular hypothesis, and finally a chapter on Saturn's rings. The text in many places will be found difficult to understand by the general reader, despite the author's efforts to fully and simply explain every point, and it seems questionable whether a thorough discussion of tidal phenomena can be made simple enough for the layman's comprehension. The volume can not be read by any one, however, without instruction, and is much the best general discussion of tidal phenomena which we have seen.



The Elementary Zoölogy of Frank E. Beddard contains an account of a few types selected from the chief groups of the animal kingdom, followed and accompanied by a consideration of some of the more general conclusions of biology. A type system has to be used, but the author has endeavored to obviate the great fault of that method—the liability of the students conceiving that the characters of the species selected for description are distinctive of a wider assemblage of forms—by emphasizing here and there the differences between allied groups. The question arises whether to begin with the higher forms and go down to the lower, which some authorities believe to be the course easier of comprehension by the student, or to follow the inverse method. The author prefers to begin with the lower forms and gradually work to the higher as the course having the undoubted advantage of presenting the facts in a logical sequence. He accordingly begins with the amoeba and proceeds upward. The treatment is simple and lucid. Novelty has not been sought in the illustrations, though there are several new ones, but selections have been made from the best already drawn.

An Introductory Logic grew out of the lectures of the author, Prof. J. E. Creighton, to undergraduate classes in Cornell University; is intended primarily as a text-book for students, and aims at being both practical and theoretical. The broad view is taken in the definition of the subject that logic is the science of thought, or the science that investigates the process of thinking; and the author expresses himself convinced