150 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



spreading out in all directions, and then gradually thinning out 

 as an upper current, extending even untp the verge of the area 

 whence the indraught is drawn? If so, does it tlien 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 questioi^.s 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 hy Mm 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 sj)pears, 

 therefore, that we here have placed in our hands a clew, which, 

 attenuated and gossamer-like though it at first appears, is never- 

 theless palpable and strong enough to gTiide 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 investi- 

 gations 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, extend- 

 ing sometimes at this season not farther from the equator than 



