RED FOGS AND SEA DUST. 117 



might hope actually to prove, by evidence the most positive, the 

 channels through which the air of the trade-winds, after ascending 

 at the equator, returns whence it came. 



270. But the air is invisible ; and it is not easily perceived how 

 either marks or tallies may be put upon it, that it may be traced 

 in its paths through the clouds. The skeptic, therefore, who finds 

 it hard to believe that the general circulation is such as Plate I. 

 represents it to be, might consider himself safe in his unbelief 

 were he to declare his willingness to give it up the moment any 

 one should put tallies on the wings of the wind, VN^hich would en- 

 able him to recognize that air again, and those tallies, when found 

 at other parts of the earth's surface. 



271. As difficult as this seems to be, it has actually been done. 

 Ehrenberg, with his microscope, has established, almost beyond a 

 doubt, that the air which the southeast trade-winds bring to the 

 equator does rise up there and pass over into the northern hemi- 

 sphere. 



272. The Sirocco, or African dust, which he has been observ- 

 ing so closely, has turned out to be tallies put upon the wind in 

 the other hemisphere ; and this beautiful instrument of his ena- 

 bles us to detect the marks on these little tallies as plainly as 

 though those marks had been written upon labels of wood and tied 

 to the wings of the wind. 



273. This dust, when subjected to microscopic examination, is 

 found to consist of infusoria and organisms whose habitat is not 

 Africa, but South America, and in the southeast trade-wind region 

 of South America. Professor Ehrenberg has examined specimens 

 of sea dust from the Cape de Verds and the regions thereabout, 

 from Malta, Genoa, Lyons, and the Tyrol ; and he has found a 

 similarity among them as striking as it would have been had these 

 specimens been all taken from the same pile. South American 

 forms he recognizes in all of them ; indeed, they are the prevail- 

 ing forms in every specimen he has examined. 



274. It may, I think, be now regarded as an established fact, 

 that there is a perpetual upper current of air from South America 

 to North Africa ; and that the volume of air which flows to the 

 northward in these upper currents is nearly equal to the volume 



