127 
and in the second case 
~ — = - -sm2^^sin(^ + X), 
Both expressions have their minima and maxima coinci- 
dent with those for the northerly components of horizontal 
force, a fact which finds its confirmation in actual observa- 
tion. 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 distinc- 
dY . 
tion; while has its maxima and minima coincident 
’ dr 
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. 
At Greenwich the maximum of northerly force takes 
place at seven p.m., the minimum at noon ; the maximum 
of vertical force takes place at seven p.m., the minimum at 
eleven a.m. 
At Bombay the maximum of northerly force takes place 
at 11 a.m., the minimum at nine p.m. ; there is a very 
decided minimum of vertical force at eleven a.m. ; but there 
is no pronounced maximum ; two minor maxima occur, one 
at six a.m. and the other at midnight. 
As far as these results go, they give an emphatic answer 
in favour of the supposition that a great part, at any rate, of 
the disturbing currents lie outside the earth’s surface, a 
view which Prof. Balfour Stewart has often supported in 
the last few years. 
The results seem to me very encouraging, and I hope 
soon to be able to make use of more material, and to obtain 
more accurate expressions for the various forces concerned. 
It would help considerably all those who, like myself, 
wish to obtain some knowledge on the subjects of terrestrial 
magnetism without the aid of a staff of computers, if the 
