internal action, if either alone was effective; and would 
thus be capable of answering the question with some defi- 
niteness ; it is vain to form conclusions on so narrow a basis 
as is afforded by a single pair of terms (together with 
another related pair) of a single degree. 
The attempt to follow the writer, even on his own ground, 
is, however, beset with considerable difficulties. On page 
127 of the paper, he writes, with reference to equations (10) 
and (11), “Both expressions have their minima and maxima 
co-incident with those for the northerly components of 
horizontal force, a fact which finds its confirmation in actual 
observation. They also give us the phase of vertical force 
to be the same for each hemisphere, and not to change as in 
the case of the horizontal force. But there is an important 
dV 
distinction ; while — has its maxima and minima co- 
dr 
incident with the maxima and minima of horizontal force 
at latitudes greater than 45^ in the equatorial regions the 
maximum of horizontal force ought to be coincident with 
the minimum of vertical force, and vice versa.” The first of 
these statements is correct only if it means that a minimum 
of vertical force coincides with either a maximum or mini- 
mum of horizontal force, and a maximum of vertical force 
does the same. The second is wrong, for whilst sin2u has 
the positive sign in the northern hemisphere and the nega- 
3 
tive sign in the southern hemisphere, — ~sin2u has, of 
course, the reverse signs. And the third statement, which 
embodies the. writer’s special test, is intelligible only with 
the help of the cases to which it is presently applied, and on 
the suppositions that he is speaking now of equation (10) 
only, and that his mind has become possessed of the wrong 
impression that sin2t6 has the same sign for ail values of u 
from 0° to 180°. 
Lastly, the writer supposes currents of electricity to cir- 
