140 PHYSICi\JL GEOGEAPHY OF THE SEA, AND ITS METEOPtOLOGY. 



probable, according to human reasoning, that such is the case; 

 and though the theoretical deductions showing such to be the case 

 be never so plausible, positive proof that they are true cannot 

 fail to be received with delight and satisfaction. Were it possible 

 to take a portion of this air, which should represent, as it travels- 

 along with the south-east trades, the general course of atmospherical 

 circulation, and to put a tally on it by which we could follow it in. 

 its circuits and always recognize it, then we might hope actually to 

 prove, by evidence the most positive, the chamiels through which 

 the air of the trade-winds, after ascending at the equator, returns 

 whence it came. But the air is invisible; and it is not easily 

 perceived how either marks or tallies may be put on it, that it may 

 be traced in its paths through the clouds. The sceptic, 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, 

 which would enable him to recognize that air and those tallies- 

 again, when found at other parts of the earth's smface. As diffi- 

 cult 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 south-east trade-winds bring to the equator does 

 rise up there and pass over into the northern hemisphere. The 

 Shocco or African dust, which he has been observing so closely, 

 has turned out to be tallies put upon the wind m the other 

 hemisphere; and this beautiful instrument of his enables us to> 

 detect the marks on these little talhes as plainly as though those- 

 marks had been written upon labels of wood and tied to the wings 

 of the wind. 



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

 Ji^Tuiefafm'^'^^^* ^^'^'^^ ^^ cousist of iufasoria and organisms whose 

 belts. habitat is not Africa, but South America, and in 



the south-east 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 spot. South American forms he recognizes in all of 

 them ; mdeed, they are the prevailing forms in every specimen 

 he has examined. It may, I think, be now regarded as an esta- 

 blished fact that there is a perpetual upper current of ah from 



