578 ME ALEX. BUCHAN ON THE MEAN PRESSURE OF THE ATMOSPHERE 



The regions of low pressure are the northern portions of the North Atlantic and 

 of the North Pacific, including portions of the continents adjoining; the belt of low 

 pressure in the equatorial regions, towards which the trade winds blow ; and the 

 remarkable depression in the Antarctic regions, which probably is subject to 

 little variation throughout the year. 



In March, pressure diminishes over Asia, the middle and south of Europe, 

 and the United States of America. Everywhere else, except in the tropics, it is 

 rising. This rise of pressure is most apparent in the temperate regions of the 

 southern hemisphere. In the north of the Atlantic it is rapidly rising, the 

 average pressure in Iceland now being 29 - G09 inches, thus showing an increase of 

 034 inch as compared with January. 



In April, the heavy lines showing a pressure above the average have now all 

 but left Asia, Europe, and the United States, and the isobars of 30 inches bound 

 a belt of high pressure which completely encircles the globe in the south tem- 

 perate zone. Pressure continues to rise in the north of the Atlantic, and to the 

 north of North America, and it is probable that a space of high pressure (at least 

 30 inches) surrounds the North Pole. In this month pressure is more equally dis- 

 tributed over the globe than in any other month ; for, excepting the Antarctic 

 Ocean, it scarcely rises anywhere above 30*1 inches, or falls below 29'8 inches. 



In May, in the north of Europe, in Greenland, and in the north of America, 

 atmospheric pressure attains the maximum of the year. Pressure continues to 

 increase over the south temperate zone, and the isobar of 301 inches now nearly 

 extends round the globe. At this time the highest pressure in the southern 

 hemisphere occurs in the south-east of Australia, where, at Deniliquin, it is 

 30185 inches. Pressure is rapidly falling over Asia and the United States. 



In June, July, and August, pressure falls in the central regions of Asia to 

 about 29-5 inches. In this season this great diminution of pressure, which may 

 be regarded as absolutely determining the summer climates of Asia, reaches its 

 lowest point. Pressure falls also in the interior of North America, where at 

 Utah, Great Salt Lake, it is only about 297 inches. The annual maximum of the 

 south temperate zone is attained in these months. The isobar of 30 - l inches 

 goes completely round the globe, and a still higher pressure prevails over the 

 south of Africa, and over those parts 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 on this account a comparison of these two groups of months is 

 very instructive. 



From this period, pressures increase over the continents of the northern 

 hemisphere, and diminish over the south temperate zone, till the distribution of 

 pressure is regained, which has been already shown to prevail during the winter 

 months. In September and October, an interesting feature of these lines is a very 



