276 NITROGEN. 



great trade-wind which constantly blows across the Atlantic and 

 Pacific Oceans from east to west within the tropics. Our keen 

 east winds in spring have a low temperature which attests their 

 arctic origin. The upper or equatorial current has its course 

 deflected by similar causes ; starting from the equator it has a 

 greater projectile force to the east than the parallels of latitude 

 over which it has to pass, and retaining this motion towards the 

 east it appears, as it passes over them, a west wind or wind 

 from the west. The upper current, flowing in the opposite di- 

 rection from the trade-wind below, was actually experienced by 

 Humboldt and Bonpland on the summit of the Peak of Tene- 

 riffe, and has been indicated at various times by the transport of 

 volcanic ashes by its means. 



On the great oceans, within the temperate zone, westerly 

 winds prevail greatly over easterly, which are supposed by some 

 to be the upper current descending to the surface of the earth. 

 These westerly winds temper the climate of the western sea-board 

 both of Europe and America, which is much milder than the 

 climate of their eastern coasts. 



The nature of the movement of the atmosphere in hurricanes 

 has lately received considerable elucidation. It appears that 

 they move in circles, and are great horizontal vortices, which 

 are probably produced by currents of air meeting obliquely, 

 like the little eddies or whirlwinds formed at the corner of streets. 

 The whole vortex also travels, but its movement of translation 

 is slow compared with its velocity of rotation.* 



The properties of the atmosphere are much affected by the 

 presence of watery vapour in it, which it acquires from contact 

 with the surface of the sea, lakes, rivers and humid soil. The 

 quantity which can rise into the air is limited by its tempera- 

 ture (page 77)> and comes to be deposited again from various 

 causes. The surface of the earth is cooled by radiation, and 

 occasions the precipitation of dew from the air in contact 

 with it. Vapour is also condensed into drops, from various 

 agencies within the atmosphere itself. The following are the 

 principal causes of clouds and rain. 1. The ascent of air in 

 the atmosphere, and its consequent rarefaction, which is attended 

 with cold. A cloud will be observed within the receiver of an 

 air-pump, on the plate of which a little water has been spilt, on 



* See the work of Colonel Reid on the Law of Storms ; and Athenaeum, August 

 25, 1838, (p. 594.) 



