382 



HYDROGRAPHY. 



Southern Europe, towards the rarefied areas of the Great Desert, 

 while the sun pours his vertical rays on it; but, on the sun's return 

 to southern declination, the Desert becomes cooled, and light airs at 

 times flow from it towards the heated waters of the Mediterranean. 

 In Egypt, a cold current of air flows from the south during the winter 

 months, towards that sea. 



A peculiarity exists in the currents of air on the Red Sea. They 

 flow in from the north and northwest ; but their limits do not extend 

 beyond half of the length of that sea, where they become neutralized 

 by the high temperature ; and, as has been observed, the currents 

 from the Southwest Monsoon of the Indian Ocean prevail in the 

 southern half. 



Although we believe that the heated waters of the Tropics, which 

 we have shown to exist, are sufficient to account for the flow of the 

 aerial currents towards the Equatorial regions, we are far from deny- 

 ing the action and influence of the sun. 



It will be seen by the map, that these heated waters occupy a much 

 greater area in the northern than in the southern hemisphere, which 

 we think is the cause of the unequal prevalence of the Trade Winds, 

 as respects their extent in latitude. It is a well-established fact that 

 the Northeasterly Trade Wind rarely if ever reaches the Equator ; 

 while the Southeast Trade, coming from the South Atlantic, passes at 

 times several degrees to the north of that line. The anomalies and 

 vibrations of the Trade Winds, we conceive, are caused by the sun's 

 heat. These have been for a long time noticed by all navigators ; 

 and we think it quite clear that the diurnal increase and decrease of 

 the Trade Winds which always follows or precedes the sun's course, 

 renders these winds fitful and unsteady. The temporary heated belt 

 and greater evaporation which is created by his rays, has an imme- 

 diate influence, causing a rarefied area to follow, as it were, in his 

 wake, into which the denser air rushes with velocity, giving rise to 

 the extension of the Trade Winds in latitude by drawing the air 

 from more remote parts, and sudden squalls, condensation, and con- 

 sequent precipitation, and not unfrequently a change of wind through 

 several points for a short time, occurs in what is supposed to be the 

 very strength of the Trade Winds. When the sun is in the opposite 

 hemisphere, the Trade Winds are more steady and regular, having 

 less of a fitful character ; and the squalls of wind and rain are rarely 

 experienced. 



The calm areas have been for a long time a terror to navigators, and 



