8SS 



HYDROGRAPHY. 



with them which they had acquired when within the lower Equato- 

 rial limits. 



To return to the surface currents. Although the map which ac- 

 companies this chapter exhibits the flow of the aerial currents in the 

 different seasons, it is necessary to state from whence these currents 

 are supplied. In order to understand their action more fully, I shall 

 endeavor to explain their flow. 



Having satisfactorily proved that the Trade Winds do not receive 

 their supply of air in a continuous current from the Poles, and that 

 the return or upper current does not exist, it is incumbent to show 

 whence they receive the vast supply and the circulation. 



Lest I may not have made the interchange of the atmosphere fully 

 understood by those who may not be familiar with the action of the 

 currents of air, I shall now explain how great Equatorial currents are 

 supplied and maintained. Prior to tracing them out, I will give what 

 I conceive to be the progression of the atmosphere around the globe, 

 from the cooler to the heated area, then ascending to the cooler upper 

 strata, and onward to the heated area again. This may be better 

 understood by the annexed diagram. 



A represents a part of the earth's surface over which the great cur- 

 rent is passing ; «, a', a" are the cold areas of the upper strata ; b, b', 



b", the heated ones of the surface ; the long arrow shows the direction 

 of the cold or dense wind passing from a to b. It then rises in the 

 direction of the short arrow to a', becomes cooled, and flows off to the 

 warm area b' ; thence it again rises to a", and follows the same alter- 

 nating course to b" ; but always towards the direction where it is most 

 strongly induced. Now, these lines are all tangents to the earth's 

 surface, and the shortest distance between the two areas. The cur- 

 rents of air, in this way, pass over a mountain chain, rising up to its 

 top, and leap over the lesser Monsoons, which flow underneath, 

 parallel to the line of the coast or mountain range. 



In examining the position of the heated areas of the ocean, we find 

 that they generally lie, within the Tropics, to the westward of the 



