RED FOGS AND SEA DUST. 
101 
" Hardly has the surface of the earth received the refreshing 
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 light, 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 Verd 
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 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 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 (§ 43) suggested by my researches, 
