528 
NATURE 
[JuLy 25, 1912 
proteins, does not give rise to putrefactive pro- 
ducts which are the basis of auto-intoxication, so 
that the diet in this sense is an ideal one. It is 
interesting to note that the author has tried again 
and again a strictly vegetarian diet, but does not 
find it so satisfactory as those mentioned. 
Making allowance for the author’s enthusiasm 
and special pleading, we may conclude that there 
“something in it.” 
(3) The first edition of this book was noticed in 
Nature of January 21, 1909. The author, by 
adopting a series of exercises carried out in bed, 
claims that he became a rejuvenated individual. 
As we said regarding the first edition, we think 
that the author has devised a system of physical 
exercises which, if carried out, would be of con- 
siderable benefit to those who, either from neces- 
sity or inclination, lead a sedentary life. 
is 
OUR BOOKSHELF. 
Lectures delivered at the Celebration of the 
Twentieth Anniversary of the Foundation of 
Clark University, under the auspices of the 
Department of Physics. By Vito Volterra, 
Ernest Rutherford, Robert Wm. Wood, and 
Carl Barus. Worcester, Mass., September 
7-11, 1909. Pp. vii+161. (Published by Clark 
University; New York and London: G. E&. 
Stechert and Co., 1912.) Price 10s, net. 
THE system of holding conferences at which a 
number of lectures are given by eminent special- 
ists is a noticeable feature of American universi- 
ties, and is being adopted with success in other 
countries. Clark University was founded in 1889, 
and under the invitaton of its Department of 
Physics courses of lectures were given to celebrate 
its twentieth anniversary. Those published in this 
volume are by Prof. Vito Volterra on recent pro- 
gress in mathematical physics (in French), by Prof. 
Rutherford on the history of the alpha rays, by 
Prof. R. W. Wood on optical properties of metallic 
vapours, and by Prof. C. Barus on physical pro- 
perties of iron carbides. Other lectures by Profs. 
Michelson and E. F. Nichols are not published. 
The volume will be of interest to those who at- 
tended the conferences or who desire a not too 
extensive summary of our knowledge in the 
branches of study covered by the lectures. 
Magnetochemie. Beziehungen swischen magnet- 
ischen Eigenschaften und chemischer Natur. 
By Prof. E. Wedekind. Pp. vilit+114. 
(Berlin: Gebriider Borntraeger, 1911.) Price 
3 marks. 
Tue subject of this monograph is one which 
has attracted considerable attention within 
recent years. It is, as the title implies, the study 
of the relation between magnetic quality and 
chemical composition. The author begins with a 
short sketch of the methods of magnetic measure- 
ment, which is useful and no doubt sufficient for 
his purpose, although it is not free from blemishes 
NO. 2230, VOL. 89] 
which one might desire to see removed. For 
instance, the diagrams on pp. 7 and 10 are ex- 
tremely rough, and even misleading. Then 
follows an account of the ferromagnetic sub- 
stances, and after this the magnetism of dissolved 
salts is described. Paramagnetism and diamagnet- 
ism are then dealt with, and the book concludes 
with a sketch of the “magneton ” theory. 
The descriptive parts are good and extremely 
useful as a record of modern work and progress; 
but the book is essentially qualitative in character 
and contains little in the way of exact analysis of 
the results which have been obtained. The work 
will be valued by those engaged in research upon 
the subject, and also by those wishing to obtain 
some general acquaintance with it. We have, un- 
fortunately, too few books of this character in our 
own language. 
The Teachers’ Book of Constructive Work for 
Elementary Schools. By Ed. J. S. Lay. Pp. 
xii+142. (London: Macmillan and Co., Ltd., 
1912.) Price 3s. 6d. net. 
EacH year now sees more attention given to 
school exercises in the various subjects of 
the curriculum which demand the employment 
of the hands as well as the brains of 
the children. Teachers of experience under- 
stand that young pupils learn best by doing, and 
this view gains ground everywhere. Mr. Lay in 
this book describes for the benefit of other 
teachers how he has succeeded in giving reality 
to lessons in arithmetic, history, geography, and 
so on, by constructive work of an interesting 
kind, so graduated that the method may be em- 
ployed with children from five to fourteen years 
of age. The book may be commended to the 
notice of schoolmasters and schoolmistresses as 
an example of what can be done with very little 
expenditure to make elementary education less 
bookish and unreal. 
LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications. | 
Forced Vibrations. 
In his letter on the above. subject in Nature of 
June 27, Prof. Perry examines some _ typical 
dynamical and electrical solutions of the equation 
(D?-+2kD--n?)x—=Bicosipt . cactn (a) 
with special reference to the critical case of maximum 
amplitude of the forced vibration; he shows that in 
all the cases examined the critical value of p which 
excites maximum response is either n or /n?—2k?, 
while the frequency of the free damped vibration is 
given by “n*—k*, and concludes that the usual 
statement is not correct that for maximum response 
“the forcing influence ought to be in tune with the 
natural frequency of the system.’’ But is it usual to 
define the natural frequency of the system as 
/n?—k*? The term is ordinarily employed, like the 
