272 Proximate Causes of certain Wituls and Storms. 



have the amount of heat communicated to the portions of air lying 

 north and south of them nearly the same, or along which the point 

 of greatest heat, or of a heat very little below the greatest may be 

 supposed to travel from east to west. If the excess of heat on one 

 side be moderately increased, the plane of the vortex will be inclin- 

 ed in that direction, but if the excess become considerable as through 

 the greater part of the temperate zone, the equilibrium will be es- 

 tablished in a totally different way. Thus with regard to the United 

 States, the point of greatest heat first passes south of us, and an im- 

 pulse is given to the under strata of the atmosphere in that direction, 

 and when sometime afterwards the columns in the meridians west of 

 us come to be expanded, the air that should have supplied the east- 

 ern or trade wind having passed off towards the equator, the upper 

 or western current descends to the earth creating a westerly wind, or 

 rather by the composition of motions in consequence of its mingling 

 with the current that is proceeding southward, a north-west wind, 

 which may be regarded as the natural wind of the parts of the globe 

 lying on the north side of the equator beyond the 30th parallel. The 

 same reasoning applies to the other hemisphere. As however the 

 natural and gende flow of the air in this direction is interrupted by 

 evaporation, condensation and other causes, the result is simply a pre- 

 dominance in those latitudes of winds from the west, and the direc- 

 tion of the pole over those from the opposite quarters. 



Two different causes therefore must exert an influence in the pro- 

 duction of the trade winds. One is the permanent elevation of the 

 temperature of the parallels lying near the equator over those more 

 remote from it. Its action is indirect and most energetic in the 

 northern parts of the temperate zone. The other is the diurnal in- 

 crease of the temperature of the earth in all latitudes in consequence 

 of the passage of the sun over the meridian. Its action is direct 

 within the limit of the trades. That it is adequate to the creation of 

 a considerable wind is proved by the fact that it is upon this that the 

 other or permanent temperature depends, and that it is what deter- 

 mines the existence of two winds ; the land and sea breezes blowing 

 in opposite directions every twenty four hours. B}^ attending to the 

 phenomena of the trade winds in different parts of the globe, we may 

 form a tolerable conjecture respecdng the one of the two causes 

 which must be supposed to exert a predominant influence in the pro- 

 duction of the total effect. In the immediate neighborhood of the 

 equator, or at a small distance on the north side of it, the cause as- 



