Page:Physical Geography of the Sea and its Meteorology.djvu/184

158 greatest at the poles of maximum cold; that it varies with the seasons, and changes night and day; nay, the atmosphere has regular variations in its electrical conditions expressed daily at stated hours of maximum and minimum tension. Coincident with this, and in all parts of the would, but especially in subtropical latitudes, the barometer also has its maxima and minima readings for the day. So also, and at the same hours, the needle attains the maxima and minima of its diurnal variations. Without other time-piece, the hour of the day may be told by these maxima and minima, each group of which occurs twice a-day and at six-hour intervals. These invisible ebbings and flowings—the diurnal change in the electrical tension—the diurnal variation of the needle,—and the diurnal rising and falling of the barometer,—follow each other as closely and as surely, if not quite as regularly, as night the day. Any cause which produces changes in atmospheric pressure invariably puts it in motion, giving rise to gentle airs or furious gales, according to degree; and here, at least, we have a relation between the movements in the air and the movements of the needle so close that it is difficult to say which is cause, which effect, or whether the two be not the effects of a common cause.

349. The question raised by modern researches.—Indeed, such is the nature of this imponderable called magnetism, and such the suggestions made by Faraday's discoveries, that the question has been raised in the minds of the most profound philosophers of the age whether the various forces of light, heat, and gravitation, of chemical affinity, electricity, and magnetism, may not yet be all traced to one common source. Surely, then, it cannot be considered as unphilosophical to inquire of magnetism for some of the anomalous movements that are observed in the atmosphere. These anomalies are many; they are not confined to the easting of the trade-winds; they are to be found in the counter-trades and the calm belts also. There is reason to believe, as has already been stated (§ 288), that there is a crossing of the winds at the calm belts (§ 212), and it was promised to go more into detail concerning the circumstances which seem to favour this belief. Our researches have enabled us, for instance, to trace from the belt of calms, near the tropic of Cancer, which extends entirely across the seas, an efflux of air both to the north and to the south. From the south side of this belt the air flows in a steady breeze, called the north-east trade-winds, towards the