and other Forces. 251 
mena of attraction, or for the currents themselves, or explain 
their continued existence.” The generation of such currents I 
have explained on hydrodynamical principles in my theories of 
galvanism and magnetism, as also in that of electricity. They 
are shown to be secondary currents, which are always produced 
when a uniform primary current traverses a medium, in which 
there is a gradation of density, such as that which must exist 
from the top to the bottom of a heavy mass resting on a hori- 
zontal plane, in order that the force of gravity on the individual 
particles may be counteracted. The primary currents are ascribed 
exclusively to motions of the ether caused by the rotations of 
the earth and of the other bodies of the solar system about their 
axes. As this is a constant cause, the streams are constant. 
The retention of an induced state of gradation of density from 
end to end, is considered to be the distinctive property of a 
magnetized bar. The observed attractions and repulsions are 
satisfactorily accounted for by the variation of the fluid pressure 
from point to point of space in the secondary currents considered 
as instances of steady motion, such variation, together with the 
dynamical effect of the currents, producing differences of pres- 
sure at different points of the surfaces of the atoms of which 
the substances attracted or repelled are supposed to consist. 
Thus the three explanations which Professor Maxwell considers 
to be requisite respecting currents to which the phenomena of 
galvanism and magnetism are attributed, are in fact given by 
my general theory quite consistently with its original hypotheses. 
The other paragraph commences thus :—‘ Undulations issuing 
from a centre would, according to the calculations of Professor 
Challis, produce an effect similar to attraction in the direction 
of a centre.” I consider that both central attraction and central 
repulsion are accounted for by my calculations. Professor Max- 
well then adduces the following objection :—“ Admitting this to 
be true, we know that two series of undulations traversing the 
same space do not combine into one resultant as two attractions 
do, but produce an effect depending on relations of phase as 
well as intensity.” This point I have considered in articles 2 
and 5 of the Theory of Gravity contained in the Philosophical 
Magazine for December 1859. There is no limitation as to 
the function W, which expresses the velocity or condensation of 
the ztherial waves, excepting that it must either be a single 
circular function, or consist of the sum of several such functions. 
Let it in general be represented by }.msin (b¢+c). Then, ac- 
cording to the theoretical calculation, the motion of translation 
given to an atom in the direction of the propagation of the 
waves, that is, the repulsive action, depends on the non-periodic 
part of the square of this function, the quantity which I have 
