the parallel of 15° north. The breadth of the calms of Cancer is also variable; so also are their limits. The extreme vibration of this zone is between the parallels of 17° and 38° north, according to the season of the year.
330. Red fogs do not always occur at the same place, but they occur on a north-east and south-west range.—According to the hypothesis (§ 210) suggested by my researches, this is the region in which the upper currents of atmosphere that ascended in the equatorial calms, and flowed off to the northward and eastward, are supposed to descend. This, therefore, is the region in which the atmosphere that bears the "rain dust," or "African sand," descends to the surface; and this, therefore, is the region, it might be supposed, which would be the most liable to showers of this "dust." This is the region in which the Cape Verd Islands are situated; they are in the direction which, theory gives to the upper current of air from the Orinoco and Amazon with its "rain dust," and they are in the region of the most frequent showers of "rain dust": all of which, though they do not absolutely prove, are nevertheless strikingly in conformity with this theory as to the circulation of the atmosphere.
331. Condition requisite to the production of a sea fog.—It is true that, in the present state of our information, we cannot tell why this "rain dust" should not be gradually precipitated from this upper current, and descend into the stratum of trade-winds, as it passes from the equator to higher northern latitudes; neither can we tell why the vapour which the same winds carry along should not, in like manner, be precipitated on the way; nor why we should have a thunder-storm, a gale of wind, or the display of any other atmospherical phenomenon to-morrow, and not to-day: all that we can say is, that the conditions of to-day are not such as the phenomenon requires for its own development. Therefore, though we cannot tell why the "sea-dust" should not always fall in the same place, we may nevertheless suppose that it is not always in the atmosphere, for the storms that take it up occur only occasionally, and that when up, and in passing the same parallels, it does not, any more than the vapour from a given part of the sea, always meet with the conditions—electrical and others—favourable to its descent, and that these conditions, as with the vapour, may occur now in this place, now in that. But that the fall does occur always in the same atmospherical vein or general direction, my investigations would suggest, and Ehrenberg's researches prove. Judging by the fall of sea or rain
