144: THE PHYSICAL GEOGRAPHY OF THE SEA. 



many in circumference, we know, and we can imagine that it is 

 like a shaft many times thicker than it is tall ; but how is it crown- 

 ed ? Is it crowned like the stem of a mushroom, with an efflo- 

 rescence or ebullition of heated air flaring over and spreading out 

 in all directions, and then gradually thinning out as an upper cur- 

 rent, extending even unto the verge of the area whence the in- 

 draught 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 

 vapor 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 ihe 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 gen- 

 eral circulation of the atmosphere. It may be in the power of 

 observation and the microscope to give light here. Let us hope. 



328. The color of the " rain dust," when collected in parcels 

 The color of ''sea- ^^^ seut to Ehrcubcrg, is "brick-red," or " yellow 

 •^"^*-" ochre ;" when seen by Humboldt in the air, it was 



less deeply shaded, and is described hy him 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 dirty-red color ; but the threads of this cocoon, 

 as seen singly in the diaphragm, were of a golden color ; there 

 would seem, therefore, no difficulty 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. 



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

 A clew leading into a clcw, which, attenuated and gossamer-like though 



the chambers of the . n • i i ^ ^ -\ ^ 



south. 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 Yerd Islands, is remarked 

 upon with emphasis by the microscopist. It is worthy of remark, 



