142 Notices respecting New Books. 
The little book, in fact, occupies a distinct niche of its own, 
between elementary popular volumes on the one hand and advanced 
or bulky treatises on the other. Not aiming at too much, it 
accomplishes what it does aim at in a sound and practical way. 
OFT ay. 
Exercises on Electrical and Magnetic Measurement. By R. H. Day, 
M.A. New Edition. Longmans, Green, and Co. (Pp. 188.) 
Tur object of this work is explained by the quotation in the 
preface from a lecture by ‘Sir W. Thomson :—“ In physical 
science a first essential step in the direction of learning any subject 
is to find principles of numerical reckoning and methods for prac- 
tically measuring some quality connected withit. I often say that 
when you can measure what you are speaking about and express it 
in numbers, you know something about it; but when you cannot 
measure it, when you cannot express it in numbers, your knowledge 
is of a meagre and unsatisfactory kind: it may be the beginning of 
knowledge, but you have scarcely in your thoughts advanced to the 
stage of science whatever the matter may be.” We have here a 
collection of 580 problems in Electricity and Magnetism; and any 
student who has worked through these examples, or who is able to 
correctly work any of them, may be satisfied that he is possessed 
of a thorough knowledge of the elementary portions of these 
sciences. The book is, however, something more than a mere col- 
lection of problems. Many of the problems, some at least in each 
section, are worked out in full, so that the book forms almost a 
treatise upon the subject. or example, in treating of Ohm’s law, 
we have a proof (possibly not altogether as simple as it might be), 
involving only simple algebra, of the condition for maximum cur- 
rent from a given number of cells, viz. that they must be so arranged 
as to have an internal resistance as nearly as possible equal to the 
external resistance. Most elementary textbooks simply state that 
this can be proved by the use of the differential calculus: but it is 
much more satisfactory to the student to have a proof given him. 
The book is prefaced by an explanation of the C.G.S. system of 
units ; and whilst most of the problems are given in the practical 
units—the volt, ampere, and ohm—yet a sufficient number are 
expressed in the theoretical units, to make sure that the student is 
able to employ either system, and understands the relationship 
between the two systems. We can thoroughly recommend this 
work as one most useful to the student. 
Numerical Tables and Constants. By Sypney Lupton, M.A. 
Macmillan and Co. (Pp. 96.) 
THIs is a small book containing a very large amount of useful 
information, and science teachers will be grateful to Mr. Lupton 
for this very useful compilation. The book will, however, very 
probably be found to be in use both too large and too small. The 
