93 GENEEAL OBSEEVATIONS ON THE WINDS. 



As a broad and primary principle, it may be affirmed that the complete 

 circulation of the atmosphere is demonstrated by the fact that the air is 

 composed of precisely the same elementary constituents in every part of 

 the world. This fact was practically demonstrated many years since by 

 the French Academy of Sciences, who had bottles of air most carefully 

 collected in all regions, and submitted to the most rigid analysis, whicli 

 failed to discover any difference whatever. It is manifest in a natural 

 sense, also, by its supporting animal and vegetable life universally in the 

 same manner. If it were not so, the air over a special region would, in the 

 course of ages, have become subject to the emanations and influences of 

 the land it covered. The same remark holds good, also, with the water of 

 the Ocean, equally universal in its definite characteristics, and from the 

 same cause, as will be shown hereafter. The manner in which this is 

 carried on is still involved in some mystery, although these difficulties are 

 disappearing before the rigid investigations which are now pursued and 

 applied to each new fact as it arises. 



(3.) In the year 1686, Edmund Halley* proposed the theory of the Trade 

 Winds and Monsoons, which is now generally received as an approximation 

 to the true solution. He afterwards altered his views, which were revised 

 and extended by George Hadley, in ITSS.f The following is a brief 

 summary of them : — 



(4.) The sun is constantly vertical over some part of the earth between 

 the Tropics, and this zone is consequently maintained at a much higher 

 temperature than the regions nearer the Poles. This heat on the earth's 

 surface is imparted to the air, which is therefore expanded, displaced, and 

 buoyed up from the surface, and the colder, and therefore denser and 

 heavier air from without glides in on both sides along the surface. The 

 displaced air, thus raised above its due level, and unsustained by any 

 lateral pressure, flows over, as it were, and forms an upper current in the 

 contrary direction, or towards the Poles; which, being cooled in its course, 

 and also sucked down to supply the deficiency in the extra-Tropical 

 regions, keeps up thus a continual circulation. 



At the Equator the velocity of the earth's rotation is 1,036 miles an 

 hour, in lat. 30° it is 897 miles, and in lat. 60° only 518 miles. Since the 

 Equator revolves so much more rapidly than the portions nearer the Poles, 

 it follows, that a mass of air flowing towards the Equator must be deficient 

 in rotary velocity, and, therefore, unable to keep up with the speed of the 

 new surface over which it is brought. Hence these currents from the 

 North and South nmst, as they glide along the surface, at the same time 

 lag or hang back, and drac) upon it in the direction opposite to the earth's 

 rotation, i.e., from East to West. Thus, from simple Northerly and 

 Southerly winds, they become permanent North-Easterly and South- 

 Easterly loinds. 



The lengths of the diurnal circles increase very slowly near to the Equator, 

 and for several degrees on each side of it hardly change at all. It follows 

 from this, then, that as these winds approach the Equator, their Easterly 

 tendency must diminish, and at the Equator must be expected to lose their 



* Philosophical Transactions, xvi., page 153. t Ibid.. 1735, page 5fl. 



