spreading out in all directions, and then gradually thinning out as an upper current, extending even unto the verge of the area whence the indraught is drawn? If so, does it then descend and return to the desert plains as an indraught again? Then these desert places would constitute centres of circulation for the monsoon period; and if they were such centres, whence would these winds get the vapour for their rains in Europe and Asia? Or, instead of the mushroom shape, and the flare at the top in all directions from centre to circumference, does the uprising column, like one of those submarine fountains which are said to be in the Gulf Stream off the coast of Florida, bubble up and join in with the flow of the upper current? The right answers and explanations to these questions would add greatly to our knowledge concerning the general circulation of the atmosphere. It may be in the power of observation and the microscope, or of the magnetic telegraph, to give light here. Let us hope.
328. The colour of "sea-dust."—The colour of the "rain-dust," when collected in parcels and sent to Ehrenberg, is "brick-red," or "yellow ochre;" when seen by Humboldt in the air, it was less deeply shaded, and is described by him as imparting a "straw colour" to the atmosphere. In the search of spider-lines for the diaphragm of my telescopes, I procured the finest and best threads from a cocoon of a dirty-red colour; but the threads of this cocoon, as seen singly in the diaphragm, were of a golden colour: there would seem, therefore, no difficulty in reconciling the difference between the colours of the rain-dust when viewed in little piles by the microscopist, and when seen attenuated and floating in the wind by the great traveller.
329. A clew leading into the chambers of the south.—It appears, therefore, that we here have placed in our hands a clew, which, attenuated and gossamer-like though it at first appears, is nevertheless palpable and strong enough to guide us along through the "circuits of the wind" even unto "the chambers of the south." The frequency of the fall of "rain dust" between the parallels of 17° and 25° north, and in the vicinity of the Cape Verd Islands, is remarked upon with emphasis by the microscopist. It is worthy of remark, because, in connection with the investigations at the Observatory, it is significant. The latitudinal limits of the northern edge of the north-east trade-winds are variable. In the spring they are nearest to the equator, extending sometimes at this season not farther from the equator than
