AND THE PREVAILING WINDS OVER THE GLOBE. 589 



a height is attained at which pressures at that level are equal; thence the air 

 will flow over, as an upper current, towards those regions which offer the least 

 resistance to its course, — in other words, where the tension at that height is least- 

 Over what part of the earth's surface is the pressure of the air least at great 

 heights ? Evidently, that region over which the air is coldest and driest near 

 the surface of the earth ; because, being thereby densest, the great mass of the 

 air is condensed or gathered together in the lower beds of the atmosphere, thus 

 leaving less air, or a diminished pressure, in the upper regions. Thus the extra- 

 ordinarily high pressure in Asia in winter will be due both to the low temperature 

 and great dryness of the atmosphere, and to proximity to the regions of low 

 pressure in the north of the Atlantic, the north of the Pacific, and in the/ 

 equatorial regions to the south ; from which it may be inferred that upper cur- 

 rents flow towards the centre of Asia, and that these upper currents compensate 

 for the drain arising from the surface currents, which flow out of this space in 

 all directions. In corroboration of this view, it is seen that while in winter the 

 winds in Tndia at low levels blow from some northerly point, at Dodabetta, on 

 the Neilgherry Hills, 8640 feet high, the mean direction of the wind during winter 

 is from about E.S.E. ; and, on the other hand, while in summer winds blow from 

 some southerly point at low levels, at Dodabetta they are almost wholly N.W. 



But by far the most striking illustration of this principle, is the high 

 pressure in summer which prevails in the Atlantic, between Africa and North 

 America. If the principle here suggested, as regulating the movements of the 

 atmosphere, be correct, the following will be the explanation of this singularly 

 high pressure:— Since, at this season, the temperature of the air resting on this 

 part of the ocean is much lower than that of the land, it follows that the ascend- 

 ing currents which rise from the heated plains of Africa, South America, North 

 America, and Europe, and from the tropical belt of calms to the south, will, on 

 reaching the upper regions of the atmosphere, flow over upon this part of the 

 Atlantic, because the temperature being comparatively low in the lower beds, the 

 air is condensed there, thus leaving less pressure in the upper regions. It may 

 also be added, that since the surface winds of this region are constantly drawing 

 away the air poured down upon it by the upper currents, extreme saturation of 

 the atmosphere cannot take place ; and hence the atmosphere is relatively cool 

 and dry. The high pressure maintained in the South Atlantic, between Africa 

 and South America in the summer of the southern hemisphere, corroborates this 

 view. 



From these considerations, it may be concluded that the winds on the sur- 

 face of the earth are approximately known from the isobaric lines, — the direc- 

 tion being from regions of high towards those of low pressure, subject to the 

 changes in the direction of the currents produced by the earth's rotation ; and 

 that the upper currents of the atmosphere may be inferred from the isobaric lines 



