3GS 



HYDROGRAPHY. 



motion of the earth. The whole theory of the winds depends upon 

 these circumstances. 



" There are two currents of air constantly flowing from the Poles 

 towards the Equator ; and, in order to supply the Trades, the air 

 which they keep in motion must return by some channel to the place 

 near the Poles. If this were not so, these winds would soon exhaust 

 the polar regions of atmosphere, and pile it up about the Equator, 

 and thus cease to blow, for want of air to make more wind of. This 

 return current, therefore, must be in the upper regions of the atmo- 

 sphere, at least until it passes over those parallels between which the 

 Trade Winds are always blowing on the surface. But it is held that 

 the particles of air from the Poles travel in the upper regions of the 

 atmosphere until they get near the parallel of 30°. Here they meet 

 with the particles which are travelling from the Equator to the Poles. 

 They press against each other, neutralize their motion, and produce a 

 calm. Thence come out two surface currents, and the air to main- 

 tain them comes by the downward current, from the superincumbent 

 air of this calm region." 



I believe I have thus given the different explanations of the 

 theories of the currents of air, excepting the theory of Dr. Halley, 

 who ascribes the Trade Wind to the current of air following the sun 

 in his diurnal motion to the west. 



These theories seem to have resulted from the phenomena of the 

 Trade Winds, which appear, from their vastness of volume, perma- 

 nency, and apparent constant direction, to be considered the active 

 and permanent cause of the circulation of the atmospheric currents, 

 instead of one of the effects which heated or rarefied surfaces would 

 naturally produce, thereby losing sight of the one great cause in which 

 all agree, viz. : that heat is the great destroyer of the equilibrium. 



The result has been, that arguments and hypotheses have been 

 adduced to account for the supply to this great current from the 

 colder regions. Currents and counter-currents, both above and below, 

 have been assigned to carry back the accumulation, and to restore the 

 lost equilibrium ; — so that the facts connected with the phenomena of 

 the Trade Winds appear to have been entirely lost sight of. 



It will be my endeavor, in the sequel, to show how these theories 

 will stand the test of facts ; and to controvert them by adducing proof 

 and reasons, that the causes which produce the effects witnessed in the 

 movements of the atmosphere are different from what these theories 



