234 Notices respecting New Books. 
the first two terms in its expansion. A more correct value consists 
A 
in taking e” itself, and since 
0 4, 
log 21 : o dl) 
te fo ‘@ ve 
= V/0,/,» 
A 
2 
¢ =€6 
tol 
it is obvious that the et factor is simply the square root of 
the ratio of two consecutive throws. The advantage of presenting 
the correction factor in this simple form is that the dullest person 
can readily be made to understand it; whereas even a quick-witted 
student frequently finds difficulty in grasping the correction in its 
‘‘loearithmic decrement” form. In dealing with the methods of 
standardizing a ballistic galvanometer, no reference is made (in 
Chapter J.) to the standard solenoid method, although this is fully 
described, later on, in Chapter IV., where, however, the curious 
statement is made that when the galvanometer is used with its 
circuit closed, iis constant cannot be found from the throw due to 
the discharge from a condenser of known capacity. Surely this is 
quite feasible, provided the proper value be given to the damping 
correction for each value of the total resistance of the galuanometer circuit. 
A full account of ampere-hour and watt-hour meters is given in 
Chapter I. ; in dealing with the testing of secondary cells, the author 
does not appear to lay sufficient stress on the uselessness of a 
single charge and discharge test. 
Chapter II. is devoted to the measurement of capacity and in- 
ductance, and contains a vast amount of useful information. 
including a number of approximate formule for calculating capa- 
cities and inductances in simple cases, and the results of the 
author’s wide experience in the practical measurement of small 
inductances and capacities. 
Chapter I1I. deals with Photometry. Here again, the author’s 
eminence as an expert in this subject retders the chapter of excep- 
tional value. It is gratifying to see Dr. Fleming’s adoption of 
Blondel’s rational system of units, and his condemnation of 
the use of the term ‘‘ quantity of light” in the erroneous sense 
generally attributed to it. In connexion with Rousseau’s diagram, 
explained on p. 321, it may be pointed out that the semi-circle 
GHEE need not be drawn so as “just to touch” the polar curve, 
but that it may be drawn with any convenient radius. The account 
of photometers is well up to date, and includes a description of 
the Matthews integrating photometer. 
Chapter LV. contains an account of Magnetic and Iron Testing, 
and in addition to the usual laboratory methods there is given a 
description of various commercial forms of magnetic testing 
apparatus, including several very recent types. 
Chapter V. is devoted to the highly important subject of Dynamo, 
Motor and Transformer Testing. In dealing with the term 
“ efficiency,’ it appears to us unnecessary to introduce “ conversion 
efficiency,” as “electrical” and ‘‘commercial” efficiencies appear 
to be amp’e for all practical requirements. The assumption under- 

