dust, we may suppose that the currents in the upper regions of the atmosphere are remarkable for their general regularity, as well as for their general direction and sharpness of limits, so to speak. We may imagine that certain electrical conditions are necessary to a shower of "sea-dust" as well as to a thunder-storm; and that the interval between the time of the equinoctial disturbances in the atmosphere and the occurrence of these showers, though it does not enable us to determine the true rate of motion in the general system of atmospherical circulation, yet assures us that it is not less on the average than a certain rate. We cannot pretend to prescribe the conditions requisite for bringing the dust-cloud down to the earth. The radiation from the smoke-dust—as the particles of visible smoke may be called—has the effect of loading each little atom of smoke with dew, ca,using it to descend in the black fogs of London. Any circumstances, therefore, which may cause the dust that ascends as a straw-coloured cloud from the Orinoco, to radiate its caloric and collect moisture in the sky, may cause it to descend as a red fog in the Atlantic or Mediterranean.
332. What is the agent that guides the air across the calm belts?—I do not offer these remarks as an explanation with which we ought to rest satisfied, provided other proof can be obtained; I rather offer them in the true philosophical spirit of the distinguished microscopist himself, simply as affording, as far as they are entitled to be called an explanation, that explanation which is most in conformity with the facts before us, and which is suggested by the results of a novel and beautiful system of philosophical research. It is not, however, my province, or that of any other philosopher, to dictate belief. Any one may found hypotheses if he will state his facts and the reasoning by which he derives the conclusions which constitute the hypothesis. Having done this, he should patiently wait for time, farther research, and the judgment of his peers, to expand, confirm, or reject the doctrine which he may have conceived it his duty to proclaim. Thus, though we have tallied the air, and put labels on the wind, to "tell whence it cometh and whither it goeth," yet there evidently is an agent concerned in the circulation of the atmosphere whose functions are manifest, but whose presence has never yet been clearly recognized, When the air which the north-east trade-winds bring down, meets in the equatorial calms that which the south-east trade-winds convey, and the two streams rise up together, what is it that makes them cross?
