174 Professor G. Piazzi Smyth's Mete&rological 



Again, as the capacity of air for moisture increases with 

 heat, and diminishes with cold, and as air feels to us to be 

 dry or damp, not according to the quantity of moisture which 

 it may contain, but upon the degree to which its capacity for 

 imbibing it has been satisfied, the current of air from the 

 pole to the equator continually becoming warmer and warmer, 

 is seeking to extract moisture from everything, while the 

 opposite current, having been completely saturated in the 

 high temperatures under the equator, is obliged to deposit all 

 those spoils, in its approach to the colder regions, when it 

 has no longer the power to hold up so much watery vapour : 

 hence a general reason why easterly winds are cold and dry, 

 and westerly are warm and moist. 



These general laws are much interfered with by the local 

 action of alternations of sea and land and other varieties of 

 surface, but the usual arrangement of the currents is, — that 

 from 30° to 10° latitude, the easterly or trade winds prevail 

 along the surface ; and from 35" to 70° upwards, the westerly ; 

 while from 30° to 35° in either hemisphere, and for several de- 

 grees upon and on either side of the equator, are belts of com- 

 paratively calm air ; the latter which is far better marked than 

 the other, is when the two trades meet, oppose each other and 

 cause much moisture to be precipitated, and rising up, are 

 gradually carried back to the poles in a westerly direction 

 over the trades below ; but descending again to the surface 

 at about 30°, begin precipitating their moisture again, either 

 on being compressed against the sides of mountain chains, 

 or as they are gradually cooled in their westerly progress 

 towards the poles, the returning current from whence is the 

 upper one, until it descends at 30°, to appear as the nascent 

 trade wind. 



This much has been advanced in part or whole by various 

 writers, but they have invariably maintained, that each trade 

 wind rising up at the equator, returns at once to its own 

 pole. Lieut. Maury, however, observes that this appears 

 a very unseemly division of the atmosphere of the world into 

 two halves, meeting but never mixing at the equator ; and 

 that in &uch a case, in course of ages we might expect to find 

 the air of either hemisphere notably different by reason of 



