Proximate Causes of certain Winds and Storms. 269 



a particular effect is attributed to the action of a certain cause ; if 

 on the reproduction of the cause, the effect fail to follow, we are to 

 conclude that there was an error in the first instance, and that the 

 original effect is to be traced to some other source. 



An attempt is however made by some of the philosophers who re- 

 ject altogether the theory of Halley, and embrace the views of Had- 

 ley, to account for the fact that die trade winds are limited by the 

 30th parallel, and that westerly winds prevail in the regions lying be- 

 yond it. It is said that the air which is rarified and ascends about 

 the equator, flows off towards the poles, — that being cooled and con- 

 densed, it at length descends to the earth, and retaining its original 

 velocity, moves eastward faster than the parallel over which it is in- 

 cumbent, producing a wind from the west.* He (Mr. Daniell ?) re- 

 marks that the restriction of the trade winds within the 30th degree 

 of latitude, can be accounted for on no other hypothesis. — Not upon 

 his principles. It may however be accounted for on different grounds. 



Now, according to this hypothesis, tjie westerly winds of the tem- 

 perate zones are a secondary result of a current flowing from the 

 equator towards the poles. They prevail at the surface of the earth, 

 and can therefore be generated only by a ground current, directed 

 from the lower towards the higher latitudes. For it will hardly be 

 contended, that the air rushing towards the poles, might occupy the 

 higher regions of the atmosphere, and communicate to the strata be- 

 low, its motion eastward, without at the same time communicating its 

 motion northward or southward. Is the existence of such a current 

 pi'obable ? We have already seen that the causes by which the 

 trade winds are produced, (according to the theory whose merits we 

 are now endeavoring to estimate) act with less energy within the 

 parallel of 30° than without it. Are we to embrace the opinion, 

 nevertheless, that these causes produce the trade winds within the 

 parallel, and also counteract the operation of stronger causes, and 

 determine the movements of the atmosphere to the distance of 30° 

 beyond, forcing back the air of the temperaie zone, notwithstanding 

 its tendency to approach the equator, into the neighborhood of the 

 pole ? This doctrine is very improbable, and has no evidence to sup- 

 port it. If there be an under current from the equator towards the 

 poles, between the latitudes of 30° and 60°, the air that is transferred 



* See Daniell's Meteorological Essays, p. 104. 



Vol. XIX.— No. 2. 35 



