196 
himself the task of devising a course which should 
guard the student against this danger. Illustra- 
tions are drawn from practical engineering, steam- 
ships, aéroplanes, motor-bicycles, turbines, &c., 
which should convince the reader of the real 
utility of mechanics, and arouse and preserve his 
interest. Text-books such as this do much to 
advance the cause of elementary mathematical 
education by enlarging the mental horizon of the 
student, and giving him a sound knowledge of the 
fundamental ideas, such as mass, force, energy, 
momentum, &c., without which any substantial 
progress is impossible. 
(5) This course of practical arithmetic, geo- 
metry, and mechanics is written for first-year 
students taking a technical course, and is intended 
to occupy rather more than a hundred hours. The 
first forty pages deal with fractions, decimals, 
ratio, percentagé, and graphs; the next sixty with 
the mensuration of the triangle, circle, and simple 
solids; and the remainder with the principle of 
the lever, centre of gravity, and the measurement 
of work. The examples are numerous, simple, 
and practical. 
(6) This small pamphlet gives an account of a 
graphical method for facilitating numerical calcu- 
lations required in connection with comparatively 
complicated formule occurring in scientific and 
engineering work. Although disclaiming any 
originality for the methods .he gives, the author 
points out that as yet they have received little or 
no attention from English writers. The theory is 
not difficult, but those whose mathematical know- 
ledge is small will find it easy to master the 
practical procedure if they study the examples 
which are worked out in great detail, although 
they may consider the nomenclature rather alarm- 
ing. : 
(7) There are few subjects which depend so 
much on the personality of the teacher for their 
success, and the interest they arouse in the student, 
as geometry. And this applies even more to its 
higher branches than to the elements. A care- 
fully-chosen course on projection and homography 
not only stimulates the mind of the pupil by the 
power and generality of its root ideas, but also 
induces an enthusiasm which ensures a remark- 
able rapidity of progress. There are two distinct 
methods of procedure open to the teacher. On 
one hand, he may base his work on an analytical 
foundation, thus making use from the start of 
imaginary and ideal elements, and so establish- 
ing the validity of general projection and the 
principle of continuity. Properties of homography 
and involution, and the idea of a one-to-one corre- 
spondence, also admit of valuable illustrations from 
analysis. Or, on the other hand, he may restrict 
NO. 2294, VOL. 92] 
NATURE 
| himself to the methods of pure geometry, and 
/of San Diego, South California, has been prepared 
by the local officer of the weather bureau, and © 
[OcTroBER 16, 1913 
exclude imaginary elements until, at an advanced 
stage, they emerge from the consideration of an 
overlapping involution. In*the treatise before us 
the author has adopted the latter method, which — 
we are inclined to think is rather more difficult — 
for the ordinary student. Its contents form a 
very comprehensive account of the projective geo-— 
metry of lines and conics up to the standard of a 
university honours degree. The author writes 
clearly, and has brought together an extremely 
interesting collection of properties; the excellence 
of the diagrams calls for special notice. We do 
not hesitate to say that those who use this book 
will gain a sound knowledge and appreciation of 
the principles of higher pure geometry. 
OUR BOOKSHELF. 
The Climate and Weather of San Diego, Cali- 
fornia. By F. A. Carpenter. Pp. xii+118+ 
plates. (San Diego: Chamber of Commerce, 
1913.) 
AN excellent little book on the climate and weather 
published by the local chamber of commerce. The 
book contains twenty-seven short chapters dealing 
partly with San Diego town and bay, partly with 
San Diego county, and partly with general factors 
in weather and climate. _ 
The characteristic feature of the climate is the 
“velo” cloud, to which the place owes its com- 
paratively low summer temperature, in spite of its 
proximity to the tropics. (The latitude and longi- 
tude might with advantage have found a place 
at the beginning of the book.) The “velo” cloud 
is a cloud of a low stratus type, which “veils” the 
sun in the morning, and usually disappears with 
the coming of the sea breeze in the afternoon. 
On the average the sun shines on 356 days of 
the year at San Diego, and the total rainfall is 
under 10 inches; at times, therefore, rain is 
earnestly desired, but we are told in illustration 
of the importance of local signs in weather-fore- 
casting that San Diego’s best-loved priest used 
to refuse to offer prayers for rain unless the wind 
had been in the south for three days. The book 
is eminently readable, and the statistical tables 
have been infused with a human interest. E. G. 
Petrographische Untersuchungen an Gesteinen des 
Polsengebietes in Nord-Béhmen. By Kk. H. 
Scheumann. Pp. vi+607-776. (Leipzig: 
B. G. Teubner, 1913.) Price 8 marks. 
Tue latest number of the Abhandlungen of the 
Koénigl. Sdchsischen Gesellschaft der Waissen- 
schaften contains a memoir by K. H. Scheumann 
on the Tertiary igneous rocks of the Polzen 
district, in northern Bohemia. These rocks are 
of the same age as those of the better-known 
Mittelgebirge, farther west, and have the same 
alkaline affinities, though there is not the same 
q 
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