172 



WINDS OF THE NORTHERN HEMISPHERE. 



to the south or south-west, and yet, all the observations from which they were com- 

 puted were taken within a few hundred miles of the African coast and Desert of 

 Sahara, a region the annual range of whose temperature must be exceedingly great. 

 The only way in which I can account for a fact so astonishing, is by supposing the 

 deflecting forces at these numbers to be secondary to the influence which we see so 

 strongly marked in Nos. 88, 89, and 90. Let us, then, first devote our attention 

 to these. 



The intense heat of the Great Desert rarefies the air exceedingly fi'om June to 

 October, inclusive, and hence the arrows of unparalleled length (Plate XII.) pointing 

 toward it during those months, the longest being longer than that Avhich represents 

 the most uniform of the trade-winds in the ratio of 104 to 89. The influence of 

 this rarefaction is sufficient to curve the powerful current of the trade-winds in the 

 manner exhibited on Plate VII. Nos. 89 and 90, and to produce the not less re- 

 markable change in No. 88, holding the current back and retarding it, so that its 

 progressive motion in the three months of July, August, and September united 

 hardly exceeds that during any one of the colder months of the year. But while 

 this is so, the trades on the western side of the Atlantic are pursuing nearly their 

 regular track, being but slightly affected by these influences. As a consequence, 

 the latter must leave, as it were, a partial vacuum behind them, which is filled by 

 air flowing in from the north-east and south-east. This will account for the seem- 

 ing anomaly of having a somewhat strong deflecting force directed toward mid- 

 ocean in the hottest part of the year, as in the numbers above referred to. And 

 yet it may be very naturally asked. Why does not the air from these parts supply 

 the Great Desert directly, instead of taking a circuitous route to supply the region 

 that supplies it? A question which, I confess, it seems difficult to answer. 



The following table, and Plate XIII. will assist in affording a clear idea of the 

 winds off" the west coast of Africa during the warmer months. The arrows show 

 the mean direction and progress of the wind in each square of 5°, for the months 

 of July, August, and September, the months when the influence of the Desert is 

 greatest. The numbers affixed to the arrows show the number of observations 

 from which they were computed, as contained in Maury's Pilot Chart of the North 

 Atlantic : — 



Longitude. 



Lat. 0° to 5°. 



Lat. 6° to 10°. 



Lat. 10° to 16°. 



Lat. 16° to 20°. 



Lat. 20° to 25°. 1 





^ 



s 





^ 



n 





X 



O 





»• 



1 





^i 



§ 







b'% 





Mean direction 





<« 



Mean direction 



°g 





Mean direction 



o ^ 



"o 



Mean direction 





"o 





ofWiad. 



^ 



of Wind. 



< f , 



^ 



of Wind. 



?P 



^ 



ofWiud. 



"S p 



fi 



of Wind. 



a R 



i 







csi: 



^ 





«(!< 



K 







15 











-"• 







g. 20 66'B. 



m 



446 



S. 12° 0' W. 



SS5 



179 

























Sfi 



1262 



S. 4 7 W. 



K2 



127K 



S. 86° 7' W. 



36 



47 



N. 19° 39' E. 



CO 



15 



N. 21° 60' E. 



73 



21 





S. 34 25 E. 



<in 



Sflfl 



S. 8 6 W. 



71 



Km 



N. 2 12 E . 



14i 



62ti 



N. 32 8 E. 



76 



456 



N. 29 29 E. 



81 



304 





S. 49 60 E. 



R2 



221 



S. 11 40 W. 



41 



8.S5 



N. 62 42 E . 



35 



604 



N. 38 61 E. 



81 



476 



N. 36 22 E. 









S. 63 26 E. 



79 



19C. 



S. 65 E. 



m 



224 



N. 69 36 E . 



39 



247 



N. 61 46 E. 



87 



192 



N. 47 67 E. 



84 



243 





S. 46 9 E. 



R4 



W! 



S. 46 39 E. 



17 



26H 



N. 67 15 E . 



40 



KOt: 



N. 55 1 E. 



86i 



149 









40 45 W. 



S. 82 42 E. 



SSi 



12,T 



S. 68 2 E. 



■MS 



262 



N. 64 47 E . 



62 



287 



N. 65 31 E. 



88 



166 



N. 68 4 E. 







45 60 W. 



S. 69 28 E. 



76 



19 



N. S9 42 E. 



54 



99 



N. 63 31 E . 



81 



184 



N.67 41 E. 



'" 



261 





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_ 



