68 
POPULAR SCIENCE REVIEW. 
Since we have shown that no distinction can logically he drawn 
between a motion of rotation and one of translation in regard to 
the behaviour of the lines of force about a magnet, but that the 
lines of force must be regarded as accompanying the magnet 
whatever the nature of its motion ( i.e . the system of force must 
be regarded as at rest relatively to the magnet ), so therefore no 
static charge can ensue by the rotation of the earth on its axis ; 
the system of force about the earth being relatively to it at rest. 
There may be one point that may be worth noticing here. The 
tides form two elevated protuberances of water which do not 
partake of the earth’s rotation, and therefore virtually rotate 
through the lines of force of the earth. For our purpose the 
tide may be regarded simply as a stationary ring or belt of water, 
within which the earth rotates. The inductive effect being de- 
pendent on relative motion (not on absolute motion), the same 
inductive effect is therefore produced upon the belt of water as 
if the earth were at rest and the belt of water revolved with the 
same relative velocity round the earth. It follows therefore that 
this belt of water will be inductively influenced in such a way 
that the parts of it lying near the equator are charged up with 
electricity of opposite sign to the parts lying near the poles. 
This may therefore constitute a true cause for a disturbance of 
the electric equilibrium, and which would not exist if it were not 
for the tides. 
VI. As a final illustration of the truth of the point con- 
tended for, it may be observed that it is an accepted fact 
that the magnet and the helix are perfectly analogous in 
their fundamental properties. Now it is well known that the 
inductive influence of currents upon each other depends on 
relative , not on absolute motion. So then when a helix (or 
simply a single circular current) rotates on its axis, the rotating 
helix produces (admittedly) upon external conductors an in- 
ductive effect, precisely the same as would be produced if the 
helix were at rest and the conductors were (conversely) made to 
revolve with the same relative velocity about the helix, or it is 
only a question of relative motion. So therefore (by analogy), 
when a magnet rotates on its axis, it produces the same induc- 
tive effect on external conductors, as if these conductors were 
themselves caused (conversely) to revolve with the same relative 
velocity about the stationary magnet, or it is here also only a 
question of relative motion. 
5. This method of viewing the case serves to simplify the 
phenomena of Electro-magnetic Eotation by correlating them 
under one cause, instead of regarding the mode of action as 
different in the two cases, first, when the magnet rotates on 
its axis, and, second, when, conversely, the portion of the 
circuit rotates about the magnet. Thus to take the example 
