S6 Professor Mitchell on tlie Trade- Winds. 



by radiation, and condensed, and 6 C being pressed on that side 

 by a force less than its own elasticity, will expand itself, and 

 create a current in that direction. The weight of A C being in 

 this way diminished ; and that of the columns on the east side of 

 it increased, A C will rise, the air at the base of the columns east 

 of it will flow in to supply its place, and a vortex be generated, 

 moving westward below, and eastward above. A new impulse 

 being given during each successive passage of the sun over the 

 meridian, a permanent east wind will be created *. 



Arguments will presently be adduced tending to render it 

 probable that the motion of the air within the limits of the trade- 

 winds is actually of the kind here represented. In the mean 

 time, it may be remarked, the above applies to such parallels of 

 latitude only as 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 the heat on one side be moderately increased^ 

 the plane of the vortex will be inclined in that direction ; but 

 if the excess become considerable as through the greatest part 

 of the temperate zone, the equilibrium will be established in a 

 totally diiferent way. Thus, with regard to the United States, 

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

 pulse is given to the under strata of the atmosphere in that di- 

 rection, and when some time afterwards the columns in the meri- 

 dians west of us come to be expanded, the air that should have 

 supplied the eastern 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 mo- 



• The Abbe Mann notices this expansion of the lower strata of the atmo- 

 sphere, which he denominates a heat tide, in a paper copied into the Philoso- 

 phical Magazine for November 1799, but does not trace its effects in the ge- 

 neration of winds. It has this in common with the tide, that it accompanies 

 the sun in his journey westward ; but, in regard to its cause, effects, and 

 the manner in which the equilibrium that has been disturbed by it is restored, 

 it differs entirely. I have to regret that it has not been in my power to con- 

 sult D' Alembert's " Researches sur les Causes general des Vents," of which, 

 however, Playfair ©bserves, that it is more remarkable for the resource and 

 ingenuity it displays in the management of the calculus, than for the physi- 

 cal conclusions to which it leads. 



