RED FOGS AND SEA DUST. 101 



" Hardly has the surface of the earth received the refreshmg 

 moisture, when the previously barren steppe begins to exhale 

 sweet odors, and to clothe itself with killingias, the many pani- 

 cles of the paspulum, and a variety of grasses. The herbaceous 

 mimosas, with renewed sensibility to the influence of hght, unfold 

 their drooping, slumbering leaves to greet the rising sun ; and the 

 early song of birds and the opening blossoms of the water plants 

 join to salute the morning." 



164. 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 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 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 

 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 trav- 

 eler. 



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 the " circuits of the wind" till we enter "the chambers of 

 the south." 



165. 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 

 Islands, is remarked upon with emphasis by the microscopist. It 

 is worthy of remark, because, in connection with the investiga- 

 tions at the Observatory, it is significant. 



166. The latitudinal limits of the northern edore of the north- 



o 



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 the parallel of 15° north. 



167. The breadth of the calms of Cancer is also variable ; so 

 also are their limits. The extreme vibration of this zone is be- 

 tween the parallels of 17° and 38° north, according to the season 

 of the year. 



According to the hypothesis (§ 42) suggested by my researches, 



