RED FOGS AND SEA DUST. 121 



imagine that it is like a shaft many times thicker than it is tall, 

 but how is it crowned ? Is it crowned like the stem ot a mush- 

 room, with an efflorescence or ebullition of heated air flaring over 

 and 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 mon- 

 soon period ; and if they were such centres, whence would these 

 winds get the vapor for their rains in Europe and Asia ? 



285. Or, instead of the mushroom shape, and the flare at the 

 top in all directions from centre to circumference, does the upris- 

 ing 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 the microscope to give light here. Let 

 us hope. 



286. The color 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 color" 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 mud-red 

 color ; but the threads of this cocoon, as seen singly in the dia- 

 phragm, were of a golden color; there would seem, therefore, no dif- 

 ficulty in reconciling the difference between the colors of the rain 

 dust when viewed in little piles by the microscopist, and when 

 seen attenuated and floating in the wind by the great traveler. 



287. It appears, therefore, that we here have placed in our hands 

 a clew, which, attenuated and gossamer-like though it at first ap- 

 pears, is nevertheless palpable and strong enough to guide us along 



throuo-h the " circuits of the wind" even unto " the chambers of 



o 



the south." 



288. The frequency of the fall of "rain dust" between the par- 

 allels of 17° and 25° north, and in the vicinity of the Cape Yerd 



