has had the heart to follow me in a preceding chapter (IV.) around with "the wind in his circuits," will perceive 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 represented by R 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 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 disbelief, 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. They 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
