41 
Chambers has failed to notice. My statement would be 
right even if Mr. Chambers’ interpretation of it was correct ; 
for though the force on a north pole changes sign by going 
across the equator, the change of vertical force also is 
reversed by crossing the magnetic equator. The maxima 
and minima of vertical force ought to be the same, therefore, 
over the whole world, except in the small poifions included 
between the magnetic and geographical equator. 
But it is a waste of time to discuss the wording of the 
sentence which I have used to interpret equations (8) ; as 
the truth of the equations can only be tested by their 
agreement, or non-agreement, with observed facts. Obser- 
vations of the vertical force variation, all agree to show that 
the third equation is true to the same degree of accuracy in 
the southern, as it is in the northern hemisphere. 
I have, in ray previous paper, only taken into account the 
term which has the full day for its period, and omitted the 
term which has a double period each day. Mr. Chambers 
blames me, in consequence, for omitting terms as important 
as those which I have taken into account. In reality, terms 
of different periods can be treated independently of each 
other. Kesults which have been obtained by taking account 
of one period, hold, of course, for that period only ; but, as 
far as that period is concerned, they cannot be affected by 
any subsequent treatment of the terms of different periods. 
I find, as a matter of fact, that the semi-diurnal terms only 
strengthen the conclusion that the disturbing force is outside 
the earth, but my calculations on this point are not, as yet, 
concluded. 
Mr. Chambers is correct in saying that I ought to have 
tested my equations by comparing them with the observations 
of the single period variation. An inspection of the curves 
is, however, sufficient to show that the maximum of the 
single period variation, must nearly coincide with the maxi- 
