ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 73 
11. It appears that so long as the sun is above the horizon of the place, 
there is comparatively little disturbance. In other words, the hours most 
habitually disturbed are before sunrise and after sunset. It is true that 
disturbances, once originated, display themselves simultaneously at dis- 
tant localities, irrespective of the hours of the day ; but the above seems 
to give probability to a conjecture that they originate in that hemisphere 
from which the sun is absent, and on those meridians which are at the 
time in the condition of greatest mean disturbance. 
12. Of known pbysical causes, the influence of sudden internal per- 
turbations analogous to those which become perceptible to our senses, as 
earthquakes and the like, seems to me the most nearly to meet the 
observed facts. They cannot be due to any atmospheric cause. Nor is it 
very probable that anything extra-terrestrial, such as solar perturbations, 
can operate with such vigour and suddenness upon otr electric circula- 
tion. That there is a sympathy or correspondence between seismic dis- 
turbance and magnetic disturbance has been often shown, but I am not 
aware that it has ever been followed up in a comprehensive way. 
That this view implies some relation between the internal perturba- 
tions referred to, and the position of the part of the globe in which they 
originate in respect to the sun, as being in the hemisphere turned away 
from him, appears to follow, but I do not see any absurdity in such a 
supposition. 
13. Since continuous automatic registration affords a means of tracing 
the correspondence of either short-time or long-time movements with 
other observed phenomena, seismic movements, solar outbursts, auroral 
discharges, and atmospheric changes, for example such as no multiplication 
of eye-observations can do, this appears to me the first use to put it to. 
Forty years of eye-observation have added enormously to our store of 
facts, but brought us little if anything nearer a theory. Is it not time to 
_ try some other line of investigation ? 
14. With respect to the behaviour of the horizontal component during 
disturbances, depending as it does upon two variables, the dip and total 
force, it is rather unsatisfactory, but we have good and extensive data, 
and whatever principle of measurement or solution is applied to, the 
declination, must, I apprehend, be extended to this element. 
15. With respect to the vertical component I doubt whether the 
available data are as yet comparable in precision with those of the other 
two elements. I saw, however, some admirable curves at Toronto, pro- 
duced by Professor Carpmael’s new instrument (I feel doubtful now 
whether they were curves of A Y or A@), which had all the character and 
freedom of those of the horizontal force, and when these have been worked 
ap and discussed we shall know a good deal more about the influence of 
disturbances in increasing or diminishing the dip and total force. 
III. By Professor Scuuster, F.R.S. 
T should like to submit to the Committee a few points to which their 
attention, in my opinion, might with advantage be directed. 
: It is now nearly fifty years since Gauss applied the method of expan- 
sion in spherical harmonics to the elements of terrestrial magnetism. He 
considered his results only as preliminary, on account of the incomplete- 
ness of the data on which he had to work. 
