566 



Notes and Correspondence. 



[July, 



cond. But of the currents whose 

 initial movements are in a meri- 

 dional direction, that on the north 

 will successively reach parallels 

 moving more and more rapidly 

 eastward, and, by lagging behind, 

 will, by the parallelogram of motion, 

 move relatively from between the 

 north and east; whilst, from the 

 reversal of conditions, the current 

 from the south will have an appa- 

 rent motion from the south-west. 

 Instead, therefore, of being mainly 

 the cause of the north-east trade- 

 wind, the Sahara must have a ten- 

 dency to produce a westerly wind 

 on its Atlantic margin ; and this is 

 borne out by the facts of the case. 

 (See ' Penny Cyclopaedia,' art. " At- 

 lantic," vol. iii. p. 27 ; and Captain 

 Basil Hall's ' Fragments of Voyages,' 

 second series, chap, vii.) 



The Sahara, therefore, consis- 

 tently employs its great power to 

 destroy the uniformity of the trade- 

 wind ; let it be submerged, and the 

 westerly winds which now prevail 

 between Cape Bojador and the 

 mouth of the Senegal will disappear 

 with it, and the N.E. trades will re- 

 ceive an accession of territory and 

 probably of intensity. 



If it were true that the Sahara 



mainly causes the N.E. trade-wind, 

 or increases its strength, the heated 

 land of India, during the interval 

 between the spring and autumnal 

 equinoxes, should augment the 

 force and confirm the uniformity of 

 the north-east trade of the Indian 

 Ocean ; instead of which it not only 

 strangles it, but actually sets up a 

 rival in the form of the south-west 

 monsoon. In fact, it produces an 

 effect which only differs from that 

 caused by the Sahara on the Atlan- 

 tic bordering the north-west of 

 Africa, in being periodical while the 

 latter is permanent. 



Assuming, therefore, that during 

 the submergence of the Sahara the 

 Atlantic was, as now, an open ocean, 

 the north-east trade wind would 

 prevail over a larger area than at 

 present ; its force would probably 

 be greater ; the Gulf Stream would 

 consequently be of somewhat 

 greater volume, or velocity, or 

 both ; and its effect would be to 

 that extent even more anti-glacial 

 than it is under existing condi- 

 tions. 



W. Pengelly. 

 Lamorna, Torquay, 

 May 1, 1865. 



