152 PHYSICAL GEOGKAPHY OF THE SEA, AND ITS r.IETEOROLOGY. 



dust, we may siipi^osc 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. AVe 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 

 brinorinf!: 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 circum- 

 stances, 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 distin- 

 guished 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, m}^ province, or that 

 of any other philosopher, to dictate belief. Any one may found 

 hypotheses if he will state his facts aud 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 ? 



