DECEMBER 1899] PRE LTIDES 443 
THE TIDES. 
The Tides Simply Explained, with Practical Hints to Mariners. By 
the Rev. J. H. S. Moxly, B.A., T.C.D. Pp. v.+151. Rivingtons: 
London, 1899. 
There are many excellent points about this little book, although we are not 
prepared to accept all the positions taken up by the author. Laplace, Airy, 
Kelvin, and G. H. Darwin, our recognised exponents of tidal theory, are severely 
dealt with, especially the last named. The author’s object is to show that the 
much-reviled ‘Equilibrium Theory ”—the theory, in fact, which is usually 
described in our elementary physical geographies—is, in a slightly modified 
form, amply sufficient to explain all tidal mysteries. Consequently he argues 
strongly against the rival kinetic or perturbation theory as it might be termed. 
In his criticism of Darwin’s dictum that a vertical force cannot produce sideways 
motion, he seems, however, to confuse vertical force with vertical pressure ; and 
there is a good deal of loose and inaccurate reasoning between pp. 38 and 40— 
reasoning which fortunately does not affect the main design of the book. In the 
constructive part of his book Mr. Moxly is decidedly at his best, and we are not 
acquainted with any clearer statement of the results that (with the assumption 
of the two tides each day) naturally follow from the equilibrium theory. In 
some of his discussions he is particularly happy, as, for example, in his account 
of the single daily tide in high latitudes, of the high tides in the Bay of Fundy, 
and other anomalies. Cy Gr K. 
A TREATISE ON CRYSTALLOGRAPHY. 
Crystallography. By W. J. Lewis, M.A., Professor of Mineralogy in the 
University of Cambridge. Cambridge Natural Science Manuals, 
Geological Series. Pp. xii.+612, 553 figs. The University Press, 
1899. Price 14s. net. 
This substantial volume is a very full and complete geometrical treatise, 
covering much the same ground as Professor Maskelyne’s ‘ Morphology of 
Crystals,” which was published by the Oxford University Press in 1895. The 
aim of the author is evidently to supply the practical needs of university 
students, to whom the drawing and calculation of crystals measured in the 
laboratory is an exercise by which a knowledge of Crystallography can best be 
obtained ; two chapters of considerable length devoted to the subject of crystal 
drawings and projections are accordingly introduced at an early stage, and 
the general description of the various classes of crystals, which occupies the 
greater portion of the book, is illustrated by abundant examples of drawings 
and calculations very fully worked out. Frequent references to crystals in 
the University Collection indicate that the book contains many original 
observations. 
The chapters on the systems and their various subdivisions are followed by 
a chapter on twin crystals, which occupies about 100 pages, and is probably 
the most extensive general treatment of this subject which has been published 
since Sadebeck’s volume on ‘‘ Applied Crystallography.” 
The book presents the appearance of a mathematical treatise, and may 
discourage the non-mathematical student, but the methods of proof employed 
will be found to be in reality simple and so expressed as to demand no advanced 
mathematical knowledge; they are consequently somewhat laboured and lengthy, 
but the more concise and elegant treatment by analytical methods is given in a 
chapter at the end of the volume. 
A short chapter on the physical properties of crystals is introduced merely 
