§']30, :i31. KED FOGS AND SEA BREEZES. 145 



because, in connection with the investigations at the Observatory, 

 it is significant. The latitudinal limits of the northern edge of 

 the northeast trade-winds are variable. In the spring they are 

 nearest to the equator, extending sometimes at this season not far- 

 ther from the equator than 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. 



830. According to the hypothesis (§ 210) suggested by my re- 

 Eed fogs do not ai- scarchcs, this is the region in which the upper cur- 

 same place, but they rcuts of atmosphcrc that ascended in the equatorial 



occur on a northeast -in t n* i i i ii 



and southwest range, calms, auQ nowed Oil to the northward and east- 

 ward, 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 Isl- 

 ands 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 show- 

 ers 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. It is true that, in the present state of our information, we 

 Conditions requisite cau uot tell why this " rain dust" should not be 



to the production of^ ..,_ 



a sea fog. 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 

 vapor which the same winds carry along should not, in like man- 

 ner, be precipitated on the way ; nor why we should have a thun- 

 der-storm, a gale of wind, or the display of any other atmospher- 

 ical 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 can not 

 tell why the "sea-dust" should not always fall in the same place, 

 we may nevertheless suppose that it is not always in the atmos- 

 phere, 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 tJian the vapor from a given part of the sea, always meet 



K 



