158 PHYSICAL GEOGllAPHY OF THE SEA, AND ITS METEOROLOGY. 



greatest at the poles of maximum cold ; that it varies witli the 

 ocasons, 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 woild, but especially in sub- 

 tropical 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. With- 

 out 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 regularl}-, 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. Tlie question raised hy 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 Dfelt 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 



