140 THE PHYSICAL GEOGRAPHY OF THE SEA. 



824. Now tlie patient reader, who has had the heart to follow 

 Putting tauies on ^® ^^ ^ preceding chapter (IV.) around with " the 

 the wind. wind in his circuits," will perceive that evidence in 

 detail is yet wanting to establish it as a fact that the northeast and 

 southeast trades, after meeting and rising up in the equatorial 

 calms, do cross over and take the paths represented by K 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 can not 

 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 southeast trades, the general course of atmospheri- 

 cal 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 ac- 

 tually 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 

 upon it, that it may be traced in its paths through the clouds. 

 The skeptic, therefore, who finds it hard to believe that the gen- 

 eral circulation is such as Plate I. represents it to be, might con- 

 sider himself safe in his unbelief were he to declare his willing- 

 ness 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 

 (lone. 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 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 instru- 

 ment of his enables us to detect the mark 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. 



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

 found to consist of infusoria and organisms whose habitat is not 



