Proximate Causes of certain Winds and Storms. 273 



signed by Halley, acts almost by itself, and the wind prevailing there 

 appears to be from the east, but much less constant and violent than 

 at some distance on either side. At those greater distances the two 

 causes conspire, and a commensurate effect is observed. It is there 

 that the trade winds rush onward with the greatest velocity. Be- 

 tween the latitudes of 30° and 60°, the two causes act in opposition ; 

 that assigned by Halley prevails, and there is a predominance of 

 winds from the west. 



That the trade winds are in fact produced by a circulation within 

 their own limits, carried on by vortices in which the motion is west- 

 ward below, and eastward above, is rendered probable by a number 

 of separate considerations. 



(a.) The definiteness of the boundary by which the trade winds 

 are limited and separated from currents flowing in an opposite direc- 

 tion, and that they commence at once in full vigor at that boundary, 

 are circumstances of great weight. " Thus in the northern Atlantic, 

 from the same limits whence the north-east trade blows towards the 

 equator, a south-west, (or rather west-south-west,) wind not uncom- 

 monly prevails in the contrary direction. So in the southern Atlan- 

 tic, from the limits of the south-east trade, the prevalent winds are 

 nearly converse, (west-north-west.) Now adverting to these winds 

 blowing contrariwise from the same limit, there is difficulty to con- 

 ceive the origin of either trade, but as derived from upper strata of 

 the atmosphere, and if that source of supply at the commencement 

 be acknowledged, there is little reason for rejecting it in the wind's 

 subsequent progress." This author, however, attributes the trade 

 winds to the " diminution of the air's specific gravity by a'bsorption 

 of moisture."* 



{b.) In the Sandwich Islands, the trade wind blows from the north- 

 east. Upon the summit of Mouna Kea,| in Hawaii, estimated to be 

 more than eighteen thousand feet in height, Mr. Goodrich, in the 



* Colebrooke's Meteorological observations in a Voyage across the Atlantic, in 

 Brande's Journal, Vol. xiv. 



t The height of this mountain appears to be a matter of great uncertainty. It 

 would be interesting to know whether the isothermal lines would strike it at the 

 same height that they do Chimborazo. Circumstances might be mentioned which 

 would have a tendency to depress, and others having a tendency to elevate them. 

 In calculating its height from the condition of the mercury in the barometer on its 

 summit, it is probable that the coefficient employed in Europe would be found inap- 

 plicable. 



