274 
Notices of Books . 
[April, 
An Introduction to the Theory of Electricity. By Linnaeus 
Cumming, M.A., Assistant Master at Rugby School. Lon- 
don : Macmillan and Co., 1876. 
This work embraces in a compact and well-arranged form many 
of the mathematical results which have been obtained by Prof. 
Clerk Maxwell and others. It commences with a chapter on 
“ Physical Units,” in which we find definitions of Velocity , 
Acceleration, Density, Force, Momentum, Moment of a Force, 
Couple. The latter is defined as follows : “ Two forces which 
are equal in magnitude and parallel, acting in opposite directions, 
but not in the same straight line, are termed a couple .” Work 
is defined as “ resistance overcome through space.” Kinetic 
energy as “ half the producft of the mass into the square of the 
velocity of a body.” The second chapter is on the theory of the 
potential. The potential at a point is defined as “the work done 
in carrying a gramme from that point to infinity.” The expres- 
sion “ difference of potential ” is frequently used in electricity, 
and its precise meaning is this : that if the potential of any two 
attracting systems be measured, the difference represents the 
work done on a unit of mass moved from one point to the other. 
Tubes of force are “ tubular surfaces bounded by lines of force.” 
The ideas developed in this chapter are next applied to certain 
experimental results ; such as the proof that electricity resides 
entirely on the surface of bodies. A number of problems both in 
Statical and Voltaic EleCtricity follow; Ohm’s Law is discussed 
at great length, and finally we have an application of some of 
the preceding laws to magnetism. The work will be welcomed 
by all students of electricity. It supplies a distinCt want, and 
supplies it well. 
A Treatise on the Kinetic Theory of Gases. By Henry William 
Watson, M.A. Oxford : Clarendon Press. 1876. 
This small work gives us, in a condensed form, the main results 
obtained during the last few years by Clausius, Clerk-Maxwell, 
and Boltzmann. It is from first to last mathematically treated, 
and will only be useful to the reader well acquainted with mathe- 
matical modes of thought. It is included within fifty-one pages, 
and embraces thirteen propositions with their proofs. We may 
take the tenth proposition as an example : — “ A homogeneous 
gas being supposed to be constituted of moving molecules with 
any given number of degrees of freedom, required to find the 
ratio of the specific heat at constant pressure to that at constant 
volume.” 
