As Professor Darwin truly says, a mathematical argument is, after all, only organized common sense; but, unfortunately, it is usually in such a highly organized form as to be beyond the intelligence of the average reader. In the present volume,[44] however, the author has wonderfully simplified a most intricate and difficult mathematical subject, and really seems to give some justification for the above generalization. The first chapter of _The Tides_ is devoted to defining them and describing methods of observation and study. The curious tidal movements in lakes, called _seiches_, which were first systematically studied by Professor Forel on the Lake of Geneva, are taken up in the second chapter; an account of Forel's work is given, and the statement made that similar researches are now under way on other lakes, notably that of Mr. Denison on Lake Huron in this country. Tides in rivers, including an account of the curious tidal phenomenon known as a "bore," are next described, the laws governing their variation and the ways in which they differ from the tides of the open sea being carefully laid down. A brief historical chapter, containing some curious extracts from Chinese and Icelandic literature, is rather instructive anthropologically than tidally. The three following sections are taken up by a study of tide generating and modifying forces, and include an interesting account of the experiments made some years ago by Dr. Darwin and his brother, in an effort to measure tidal forces by means of the bifilar pendulum, which is now such an important agent in seismological investigation. Chapters IX and X give an account of the equilibrium, and the dynamical theories of the tide-generating forces, and are chiefly accounts of the devices by which mathematicians have endeavored to bring artificial order out of the actual chaos. The great complexity of this portion of the subject; the variety of forces operating to produce the tides, the sun, the moon, the earth's rotation, etc.; and the number of retarding and confusing elements, friction, interposed land masses, river currents, air movements, depth of water, etc., render these theories practically valueless for use in tidal calculations.