KED FOGS AND SEA BKEEZES. 151 



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. 



;]30. Bed fogs do not always occur at the same place, hut they occur 

 on a north-east and south-west range. — According to the hypothesis 

 (§ 210) snggested by my researches, this is the region in which 

 the upper currents of atmosphere that ascended in the equatorial 

 calms, and flovs^ed off to the northward and eastward, are sup- 

 posed to descend. This, therefore, is the region in which the 

 atmosphere that bears the "rain dust," or "African sand," de- 

 scends 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 Yerd Islands are 

 situated; they are in the direction which, theory gives to the 

 upper cuiTent of air from the Oiinoco 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 

 l^asses 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 v/hy 

 Ave 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. There- 

 fore, 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 j^assing 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 the.se 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 bv llio fill of i-ca or rain 



