ADDRESS. lait 
agency by which they are produced. They formed accordingly the subject 
of a distinct recommendation from the British Association, which met with 
an equally favourable reception. To investigate these variations by suitable 
instruments and methods, to separate each from the others, and to seek its 
period, its epochs of maximum and minimum, the laws of its progression, and 
its mean numerical value or amount, constituted the chief purposes for which 
magnetic observatories were established for limited periods at certain stations 
in Her Majesty’s dominions, selected in the view that by a combination of 
the results obtained at them, a general theory of each at least of the principal 
periodical variations might be derived, and tests be thus supplied whereby 
the truth of physical theories propounded for their explanation might be 
examined. We are just beginning to profit by the collocation and study of 
the great body of facts which have been collected. Variations corresponding 
in period to the earth’s revolution around the sun, and to its rotation around 
its own axis, have been ascertained to exist, and their numerical values ap- 
proximately determined in each of the three elements, the Declination, In- 
clination, and Magnetic Force. We unhesitatingly refer these variations to 
the sun as their primary source, since we find that in whatever part of the 
globe the phenomena are observed, the solstices and equinoxes are the cri- 
tical epochs of the variation whose period is a year, whilst the diurnal varia- 
tion follows in all meridians nearly the same law of local solar hours. To 
these unquestionable evidences of solar influence in the magnetic affections 
of the earth, we have now to add the recently ascertained fact, that the mag- 
netic storms, or disturbances, which in the absence of more correct know- 
ledge were supposed to be wholly irregular in their occurrence, are strictly 
periodical phenomena, conforming with systematic regularity to laws in which 
the influence of local solar hours is distinctly traced. 
But, whilst we recognise the sun as the primary cause of variations whose 
_ periods attest the source from whence they derive their origin, the mode or 
_ modes in which thé effects are produced constitute a question which has been 
_ and may still be open to a variety of opinions: the direct action of the sun 
as being itself a magnet, its calorific agency occasioning thermo-electric 
_ and galvanic currents, or in alternately exalting and depressing the magnetic 
_ condition of substances near the surface of the earth, or in one of the consti- 
_ tuents of its atmosphere,—have been severally adduced as hypotheses afford- 
ing plausible explanations. Of each and all such hypotheses the facts are the 
_ only true criterion ; but it is right that we should bear in mind that in the pre- 
sent state of our knowledge, the evidence which may give a decided counte- 
nance to one hypothesis in preference to others does not preclude their possible 
coexistence. The analysis of the collected materials and the disentanglement 
of the various effects which are comprehended in them, are far from being yet 
complete. The correspondence of the critical epochs of the annual variation 
with the solstices and equinoxes rather than with the epochs of maximum 
| and minimum temperature, which at the surface of the earth, in the subsoil 









