Page:Scientific Memoirs, Vol. 2 (1841).djvu/107

Rh and return to facts. By far the greater number of the anomalies are found to be smaller at the southern stations, and larger at the northern. For instance, the remarkable ascent of the curve, on the 30th January, 1836, between 9h 25m, and 9h40m, amounted in Catania to 6′ (reduced to parts of arc); in Milan to 12′; in Munich to 13 ½′; in Leipzig to 16′; in Marburg to 20′; in Göttingen to 26′; and at the Hague to 29′. Something, it is true, must be deducted from this inequality, due to the circumstance that, at the northern points (where the horizontal portion of the terrestrial magnetic force has a weaker intensity than at the more southern ones,) similar disturbing forces must produce greater effects; but the difference of the horizontal intensities at the Hague and Catania is very small in comparison with the inequalities observed; and it is therefore certain that the energy of the disturbing force was weaker the further we follow its action towards the south. With all the uncertainty under which we labour with respect to the nature of such disturbing forces, we cannot doubt that they have some definite source in space; and, as we must necessarily suppose those which produced the above-mentioned phænomena to have their seat to the north or to the north-west of the places of observation, (without venturing to define more precisely from so few data,) the northern districts, as far as we may venture to draw any such conclusions from experiments which embrace but a comparatively small portion of the earth's surface, appear to be the great focus, from whence proceed the greatest and most powerful actions.

A closer inspection of the data hitherto collected leads us to recognise, in the different successive movements, considerable variations in respect to their proportional magnitudes at different places, even when the similarity in other respects is unequivocal: thus, for instance, at one place, the first of two movements, following one shortly after the other, is the largest; at another place the reverse happens. We are therefore compelled to admit that, on the same day, and in the same hour, various forces are contemporaneously in action, which are probably quite independent of one another, and have very different sources; and the effects of these various forces are intermixed, in very dissimilar proportions, at various places of observation, relatively to the position and distance of these latter; or these effects may pass one into the other, one beginning to act before the other has