102 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
this is the zone 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 
zone in which the atmosphere that bears the "rain dust," or "Af- 
rican sand," descends to the surface ; and this, therefore, is the 
zone, it might be supposed, which would be the most liable to 
showers of this "dust." This is the zone in which the Cape Verd 
Islands are situated ; they are in the direction which theory gives 
to the upper current of air from the Oronoco and Amazon with 
its'" rain dust," and they are in the region of the most frequent 
showers of " rain dust," all of which are in striking conformity 
with this theory as to the circulation of the atmosphere. 
It is true that, in the present state of our information, we can ' 
not 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 vapor 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 dis- 
play 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. 
168. Therefore, though we can not tell why the sea dust should 
not fall always 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 always meet with the conditions — elec- 
trical and others — favorable to its descent, and that these condi- 
tions might 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 re- 
searches prove. 
169. Judging by the fall of sea or rain dust, we may suppose 
that the currents in the upper regions of the atmosphere are re- 
markable for their general regularity, as well as for their general 
direction and sharpness of limits, so to speak. 
We may imagine that certain electrical conditions are necessary 
to a shower of " sea dust" as well as to a thunder-storm ; and 
that the interval between the time of the equinoctial disturbances 
