Proximate Causes of certain Winds and Storms. 271 



be the lower stratum of the atmosphere, three or four miles in thick- 

 ness. Let the sun be vertical at A. The lower part of the column 

 AC will be heated and expanded, and the portion BJ lifted into the 

 position £C, undergoing at the same time a slight condensation.* 

 The portion GC will therefore have a tendency to flow over into the 

 columns on each side of it. But it cannot flow in the direction CE, 

 because the sun, moving in the direction AG at the rate of a thou- 

 sand miles an hour, and carrying the point of greatest heat forward 

 with the same velocity : before the part (C has time to yield to the 

 impulse of elasticity and gravity, arid flow into the columns west of 

 it, they will themselves have been heated and expanded, and brought 

 into the same condition with the column AC. It cannot flow either 

 north or south, or at least its tendency to escape in those directions 

 will be feeble, because all parts of the same meridian will be heated at 

 the same time. There remains therefore only the direction CD. But 

 the sun having already passed over the columns on the eastern side of 

 AC, and they being cooled by radiation, and condensed, and <*C be- 

 ing 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 

 AC being in this way diminished, and that of the columns on the 

 east side of it increased, AC will rise, the air at the base of the col- 

 umns east of it will flow in to supply its place, and a vortex be gene- 

 rated, moving westward below, and eastward above. A new im- 

 pulse being given during each successive passage of the sun over the 

 meridian, a permanent east wind will be created. f 



Arguments will presently be adduced tending to render it proba- 

 ble 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 that the above applies to such parallels of latitude only as 



* See remarks connected with Fig. 1. 



1 The Abbe Mann notices this expansion of the lower strata of the atmosphere, 

 which he denominates a heat tide, in a paper copied into the Philosophical Maga- 

 zine for Nov. 1799, but does not trace its effects in the generation of winds. It has 

 thb 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 consult D'Alembert's M Recherches sur les causes general 

 des Vents," of which, however, Playfair observes that it is more remarkable for the 

 resource and ingenuity it displays in the management of the calculus, than for *h© 

 physical conclusions to which it leads. 



