DISCUSSION AND ANALYSIS OF WINDS. 663 



determining the summer climate of Asia, reaches its lowest point. Pressure falls 

 also in the interior of North America, where, at Salt Lake City, it is only 29.7 

 inches. The annual maximum of the south temperate zone is attained in these 

 months. The isobar of 30.1 goes entirely round the globe, and a still higher pres- 

 sure prevails over South Africa, and the portions of the ocean immediately to the 

 west and east of it. In these months the arrangement of the isobars may be 

 regarded as being, generally speaking, reversed from that of December, January 

 and February, and in this respect a comparison of these two groups of months is 

 very instructive. 



" From this period, pressures increase over the continents of the northern hemi- 

 sphere, and diminish over the south temperate zone, till the distribution of pressure 

 is regained which has been shown to prevail during the winter months. 



" In September and October an interesting feature of these lines is a very rapid 

 diminution of pressure, indicated as taking place in the north of the Atlantic and 

 surrounding regions. This is the season of the year when the first great decrease 

 of temperature takes place, which is accompanied by heavy rains and furious 

 storms. The increase of pressure in Sweden in October, taken in connection 

 with the simultaneous decrease in Greenland, Iceland, the north of Norway, and 

 the British Islands, is interesting as bearing on the transport of masses of the 

 atmosphere from one region into another. 



" In November, pressure rises considerably over the continents of the northern 

 hemisphere, and falls in the south temperate zone. And the belt of low pressure 

 in the equatorial regions may be regarded as passing completely around the 

 globe. This belt, towards which the trades on each side of the equator blow, 

 does not occur in the summer months in the Indian Ocean; but, on the contrary, 

 there is a continuous diminution of pressure northward, from Australia and Mau- 

 ritius to the interior of Asia. It will be seen that in November, as compared 

 with October, the isobars have advanced a little northward from the British 

 Islands to Iceland, and eastward from Baffin's Bay to Iceland, thus indicating a 

 general increase of pressure over the north of the Atlantic and regions adjoining. 

 Coincident with this increase of pressure, there occurs a diminution of pressure to 

 the southeast of it, including Austria, Italy, and countries adjoining the Mediter- 

 ranean ; and in the Atlantic to the south of it, from about latitude 15°-45° N. 

 Probably these extensive oscillations of pressure are part of a general movement 

 of the atmosphere, which, in one of its manifestations, has been generally known 

 to meteorologists as the great November wave, but of which no very satisfactory 

 account has yet been given," (Buchan, p. 577-579.) 



Winds within, or near, a space of Loio Pressure. — " Of this class, the best exam- 

 ple is the low pressure which prevails in the north of the Atlantic and adjoining 

 regions in the winter months. This region of low pressure is bounded to the S. 

 W. by the high pressure of North America, to the S. by the high pressure in the 

 Atlantic, about 30° lat. N., to the S. E. by the high pressure in the interior of 

 Asia. In January, the difference between the average pressure of Iceland and the 

 interior of Asia is fully an inch." 



"It is seen from the charts that in Baffin's Bay and east of tlie Rocky INIountains, 



