144 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



questions 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 hght here. Let us hope. 



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

 The colour of "sea- : and seut to Ehreuberg, is "brick-red," or "yellow 

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

 less deeply shaded, and is described hy hini as imparting a " straw 

 •colom^ " to the atmosphere. In the search of spider-lines for the 

 diaphragm of my telescopes, I procui'ed 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 colom^ ; 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. It aj^pears, therefore, that we here have placed in our hands 

 A clew leading into a clow, which, attenuated and ejossamer-like thout^h 

 souLh. it at first appears, is nevertheless palpable and 

 strong enough to guide us along through the " circuits of the 

 "wind " even imto " 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, 

 because, in connection with the investigations at the Observatory, 

 it is significant. The latitudmal limits of the northern edge of 

 the north-east trade-wmds are variable. In the spring they are 

 nearest to the equator, extending sometimes at this season not far- 

 ther fi-om the equator than the j)arallel of 15° north. The breadth 

 of the calms of Cancer is also variable ; so also are then' limits. 

 The extreme vibration of this zone is between the parallels of 17° 

 and 38° north, according to the season of the year. 



330. According to the hypothesis (§ 210) suggested by my 

 Says^oSurrihe" r^searches, this is the region in which the upper 

 same place, but they curreuts of atmosplicre that ascended in the equa- 



occiir on a north-east , • ^ ^ -, ^n TiY'j_j_i j_i t'^t 



and soutii-west range, tonal caims, anci liowecl on to tne nortnwarcl and 

 eastward, are supposed to descend. This, therefore, is the region 

 in which the atmosphere that bears the "ram dust," or "African 

 sand," descends to the surface ; and this, therefore, is the region, 

 it might be supposed, which would be the most liable to showers 

 of this "dust." This is the region in which the Cape Yerd 



