168 OBSEEVATIONS ON THE WINDS. 



in the Gambia than in any other part of West Africa. This opinion, I may 

 add, is based upon nearly 14 years experience gained in Sierra Leone, the 

 Gold Coast, and the Gambia. < 



(95.) Commander B. G. Bourke, E.N., says that the haze (mentioned 

 before) is prevalent during the whole season, so that whether the Har- 

 mattan be felt on the surface of the earth or not, it is probably going on in 

 the upper regions of the atmosphere. By the motions of the upper clouds, 

 the N.E. Trade seems to be always blowing over the Northern part of 

 Western Africa, and it may be that the Harmattan is only the N.E. Trade 

 extending itself to the surface from unusual strength, or other at present 

 unknown cause. I observed sheet lightning every evening during the con- 

 tinuance of this wind, but only to seaward, which was doubtless due to the 

 rapid evaporation from the surface of the sea owing to its being in contact 

 with such very dry air.* 



7.— WINDS AND CALMS ON THE TROPIC. 



(96 ) Between the N.E. Trades and the Westerlj' winds which prevail 

 more or less to the Northward of them, there is a belt of variable and light 

 winds, which have, perhaps somewhat vaguely, been called the Calms of 

 Cancer— Qj term which will not express its characteristics. 



It is called, also, the Horse Latitudes, from the fact that vessels in former 

 years, employed in carrying horses to the West Indies, were frequently 

 obliged to throw them overboard during the embarrassment caused by the 

 continual changes, sudden gusts and calms, rains, thunder and lightning, 

 which are general in it (42), page 124. 



(97.) This zone is caused by the uniting, or interchanging, of those upper 

 but contrary currents which pass Northwards over the N.E. Trades in con- 

 sequence of the heat acquired under the tropical sun having reached the 

 Northern extreme of this superheating influence. They here meet the 

 currents passing Southwards to feed the Trades from the polar regions, and 

 thus pressing against them cause the high barometer peculiar to this belt, 

 standing as it does at a higher level than either to the North or South of it. 

 Captain Maury infers that the mean height of the mercmy in this belt is 

 30-21 inches, and at the Equator 2993 inches. Admiral FitzEoy states 

 the mean height of the barometer in the latitude of England to be 29 95 

 inches. This greater height of the mercury, showing increased pressure, 

 will be an index to the sailor that he has reached this intervening beU 

 between the Passage and Trade Winds. 



• Journal of the Meteorological Society, vol. iv., page 26. 



