119 
On the Diurnal Period of Terrestrial Magnetism,” by 
Arthuk Schuster, F.R.S. 
The explanation of the daily variation of the magnetic 
forces observed on the surface of the earth will, in all proba- 
bility, lead to the explanation of the mysterious connexion 
between solar phenomena and terrestrial magnetism. For 
the increase in amplitude of the diurnal variation of the 
horizontal components of magnetic force forms one of the 
most striking effects accompanying the increase in sunspot 
activity. The daily variation, then, seems a most important 
symptom of solar influence, and its investigation becomes a 
matter of great interest. 
In the remarks which I wrote out for the Report of the 
Committee appointed by the British Association, for the 
purpose of considering the best means of comparing and 
reducing magnetic observations, I pointed out the impor- 
tance of adopting a suggestion, made already by Gauss, to 
apply the analysis of surface harmonics to the diurnal oscil- 
lations. It is well known that such an analysis would allow 
us to decide the question whether the immediate cause of 
the disturbance was inside or outside the surface of the 
earth - nor can there be two opinions as to the importance 
of definitely settling that question. At the time I wrote 
out my suggestions, however, it seemed to me that, as the 
causes of the disturbance had their seat in all probability 
close to the surface, whether outside or inside, that we 
should require a large number of terms in the expansion 
before we could arrive at a definite result. 
In this I was mistaken, and it is one of the principal 
objects of this paper to show that the periodic variations 
adapt themselves with great facility to the analysis, and 
that even with the very limited quantity of material at our 
disposal, we shall be able to arrive at most important 
results : results which, within a short time, might be made 
absolutely certain, if additional observations were taken at 
