VARIATION OF TERRESTRIAL MAGNETISM. 
511 
It will be interesting to follow out in future researches the field wliich this investi¬ 
gation has opened, especially in order to trace the effect of the Sunspot variation ; but 
for this purpose it is absolutely necessary that different observatories should follow a 
more uniform plan in reducing their observations. It has been found by experience 
that if the hourly readings of the magnetic needle are collected together, and their 
mean taken, that mean is different according as the disturbed days are taken into 
account or rejected. In other words, the disturbances are not irregularly distributed, 
but have a daily period which is mixed up with the regular daily variation. If we 
want to separate the investigation concerning the regular variation and the disturbance 
variation, we must adopt some plan of obtaining the one without the other. I need 
not here describe Sabine’s well-known method of doing this. Grave objections have 
been urged against it, but it is still adopted in many observatories. A discussion of 
the various methods of reduction which have been proposed will be found in recent 
Eeports of the British Association, and amongst them that adopted by Mr. Wild at 
St. Petersburg seems to me to be the only one which can be justified on strict scientific 
principles. It consists in selecting the curves for the quiet days, of which there are 
always a sufficient number in each month, and not to take account, as Sabine’s 
method does, of any reading at all during the disturbed days. We get in this wmy 
something perfectly definite, namely, the mean variation of the magnetic needle 
during certain specified days. It seems to me that if the heads of different observa¬ 
tories could adopt some system of intercommunication, by which they could select 
those days which are most quiet all over the world, and if the elements are reduced 
for those days solely at the different stations, we should obtain a series of values for 
different points of the Earth which are strictly comparable with each other. The 
labour of reduction, as far as I can judge, would thereby be seriously diminished. 
The method hitherto adopted at Greenwich is very similar to that of Wild, and will 
not, probably, lead to results which are sensibly different. 
The reduction of the observations by spherical harmonic analysis would be a very 
simple matter according to the method which I have followed, if the results of different 
stations were published in a manner which would lend itself easily to the wmrk. The 
method of publication adopted at Greenwich is very convenient, and might serve as a 
model to other observatories. Much labour is, however, involved in reducing varia¬ 
tions in horizontal force and declination to variations in force towards the geographical 
West and North respectively. If all observatories could publish the coefficients of the 
harmonic series of the elements as at Greenwich, but reduced to the geographical 
instead of the magnetic co-ordinates, the progress of magnetic science wmuld be much 
assisted, as every scientific investigation must take the geographical components for 
its starting point. 
I have tried to form an idea as to the degree of accuracy reached in the deter¬ 
mination of such quantities as the daily variation of declination ; the result is not 
