74 
WA TURE 
[May 27, 1886 
ELECTRICITY TREATED EXPERIMENTALLY 
Electricity Treated Experimentally. By Lineus Cum- 
ming, M.A. (London: Rivingtons, 1886.) 
HOSE who are acquainted with Mr. Cumming’s 
“Introduction to the Theory of Electricity” will 
welcome most heartily a new and excellent little work 
from his pen. The book before us is on “ Electricity 
Treated Experimentally”; and it is highly to be com- 
mended. It is admirably clear and concise, and at the 
same time the information is full and is well arranged; 
while the multitude of excellent illustrations and the open 
double-leaded type make the little book very pleasant 
and satisfactory reading. 
The portions devoted to magnetic and electric measure- 
ments, both electro-static and electro-kinetic, are, as we 
should expect from the author, clear and full; while the 
descriptions of the various measuring instruments are 
very satisfactory. An excellent account is also given of 
Faraday’s experimental investigations in electro-statics 
and electro-magnetism, and of those of Ampere in electro- 
dynamics. 
The least satisfactory portion of the book is the 
chapter headed “Current Induction.” This chapter, 
even making all allowances for its necessary brevity, re- 
quires very considerable improvement and amendment. 
The descriptions given of dynamo-electric machines are 
very far from adequate, even to the extent of making 
little or no distinction between a magneto-electric machine 
and a so-called “ dynamo.” Under the heading “ Sie- 
mens Dynamo” there is a description and diagram of 
the old Siemens shuttle-wound armature; and Fig. 218, 
which is a diagram of aGramme magneto, shows the soft 
iron of the armature cut away almost to nothing to make 
space for the armature. The information given with 
respect to the incandescent lamps and incandescent 
lighting also requires improvement to make it suitable for 
the present day ; and the description of the telephone 
and of experiments to illustrate the action of it are not 
satisfactory. Some of these instruments it is perhaps un- 
necessary to treat of in a book of this class; but if they 
are dealt with at all the treatment must be correct and 
not too meagre. 
One or two other minor matters we cannot avoid men- 
tioning. The first is the naming of the magnetic poles. 
It is greatly to be desired that strong efforts should be 
made by all teachers to get rid of the English “north” 
and “south.” Most writers of importance are doing this 
now; either by adopting “blue” and “red” for ¢rue 
north and /rwe south respectively, or else by using in 
full the designations “true north” and “true south.” 
However this may be, the practice of marking the ends 
of a magnet + and — seems to us thoroughly objection- 
able. 
Next we would call the author’s attention to the fact 
that the rule which he has called Oersted’s rule for 
finding the direction in which a magnet turns under the 
influence of a current is commonly, and we believe 
rightly, called Ampére’s rule. But it would be of very 
great advantage if Ampére’s rule were improved out of ex- 
istence, and some such rule substituted as that “ terrestrial 
currents sz/pused to correspond with terrestrial magnetism 
follow the sun.” When the unfortunate student imagines 
himself lying on his face, or (?) back, with a current entering 
by his feet, or (?) head, and stretches out his right hand, 
or (?) left, to show the direction of the deflection of the 
magnet, the probabilities against his coming at the end 
of his imagining to a correct conclusion are considerable. 
It seems strange that sucha rule should have held its 
place from Ampére’s time till now. 
Lastly, we miss the name of Cavendish and his proof 
(by means of the experiments of Faraday so well de- 
scribed) of the electro-static law of the inverse square of 
the distance. It is impossible, by means of the torsion- 
balance, to give anything but a rough proof of this great 
Jaw. But Cavendish established mathematically that no 
other law than that of the inverse square of the distance 
will account for the whole electric charge being found on 
the outside of a closed conductor ; while the experiments 
of Faraday established to minute accuracy this celebrated 
law of electric distribution. In searching for the name 
of Cavendish, too, an alphabetical index would have been 
of much assistance. It is sad for a reviewer to take up a 
book without an index! No book, unless it be a novel, 
should be without one. For small books it is easily made; 
for large books it is essential. 
With these criticisms we must take our leave of Mr. 
Cumming’s book ; but we cannot do so without remark- 
ing once more that it is one of the pleasantest and most 
thorough little books on electricity and magnetism with 
which we are acquainted. Ney tli 183, 
OUR BOOK SHELF 
Constructive Geometry of Plane Curves. With Numerous 
Examples. By T. H. Eagles, M.A. Pp. xx, 374° 
(London: Macmillan and Co., 1885.) 
THIs book differs considerably from previous treatises on 
practical geometry. The author has made a serious 
attempt to improve the instruction usually given in his 
subject, and the result is that we have a text-book which 
will lend itself to class-teaching of a thorough and 
searching character. 
Hitherto much time has been spent on constructions 
which furnish no mental discipline. In this treatise the 
proofs of the methods used are given or indicated in 
every case. 
A valuable collection of examples is supplied at the 
end of each chapter. If a numerical result is involved, 
the answer is usually appended, and hints are given 
towards the solution of the more difficult examples. 
Two-thirds of the book is devoted to conic sections, 
and herein we find methods of drawing these curves under 
almost any conceivable conditions ; there are also chapters 
on reciprocal polars and the anharmonic properties of 
conics which will give the draughtsman some indication 
of the power of modern geometry and of its usefulness in 
practical application. 
After a chapter on conics as derived from plane sec- 
tions of a cone, we have about 100 pages devoted to 
various other curves which are of interest in mechanics 
or physics. Compared with the exhaustive treatment of 
the conic sections, the account of several of these curves 
is somewhat scanty. 
We should like to see more space given to equipotential 
curves, for instance, and to have further exemplification 
of the methods of construction adopted by Rankine and 
Maxwell. 
The book closes with an interesting chapter on the 
graphical solution of quadratic equations and certain 
trigonometrical equations. 
a 
