l-iC rnTSICAL GEOGRArHY OF THE SEA, AND ITS METEOROLOGY. 



has had the heart to follow me in a preceding chapter (IV.) 

 around with " the wind in his circuits," will j^erceive that 

 evidence in detail is yet wanting to establish it as a fact that the 

 north-east and south-east trades, after meeting and rising up in 

 the equatorial calms, do cross over and take the paths repre- 

 sented by E S and F G, Plate I. Statements, and reasons, and 

 arguments enough have already been made and adduced (§ 288) 

 to make it highly 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 channels 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 circu- 

 lation is such as Plate I. represents it to be, might consider him- 

 self safe in his mibelief, 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 

 surface. 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 south-east trade-winds 

 bring to the equator does rise up there and pass over into 

 the northern hemisphere. The Sirocco or African dust, which 

 he has been observing so closely, has turned out to be tallies put 

 upon the wind in the other hemisphere; and this beautiful 

 instrument of his enables 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. 



325. TJiey tell of a crossing at the calm belts. — 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 south-east trade-wind region of South 



