REPORT ON ATMOSPHERIC CIRCULATION. 59 



In this month the pressure of the northern hemisphere, taken as a whole, falls to 

 the annual minimum. If 29 - 95 inches be accepted as the mean pressure of the atmo- 

 sphere over the globe, then the whole of this hemisphere, excepting the anticyclonic 

 regions of the Atlantic and Pacific, has a mean pressure below the average. This great 

 seasonal depression has its centre marked off by the isobar of 29 - 40 inches, extending 

 from Mooltan to Muscat, and is absolutely the lowest continental pressure occurring 

 anywhere or at any season. This great depression, which may be roughly regarded as 

 coterminous with the land of the northern hemisphere, may be justly considered as 

 ruling the climate and weather of this half of the globe during the summer months. 



Subordinate centres of low pressure are to be seen in North America, between 

 South Greenland and Hudson Bay, south of Iceland, in Scandinavia, in Spain, and in 

 the valley of the Po, the last four being, however, comparatively slight. In America the 

 lowest isobar is 29 "75 inches. In Africa the increased heat seems to result in a widen- 

 ing apart of the isobars from the Red Sea to Sierra Leone, rather than in the formation 

 of any distinct cyclonic centre. 



In this month pressure in equatorial Atlantic, between the anticyclonic regions 

 north and south of it, reaches its annual maximum, not falling as low as 29 '90 inches. 



In addition to the four anticyclones in the Atlantic and Pacific, anticyclones appear 

 also to the west of Australia, in South Africa, and in Australia, in the last case reaching 

 the maximum for the year. In the southern hemisphere, about lat. 30°, pressure rises 

 over long stretches to or above 30 "20 inches ; and nowhere, except in the comparatively 

 short distance from long. 170° E. to long. 140° W., does it fall below 30'00 inches. It 

 is to this belt of high pressure that the part of the air which has been removed from the 

 continents of the northern hemisphere has been transferred. 



In January the highest mean pressure in Asia is a little more than 30 '50 inches in 

 the upper valley of the Amur and the region to the south-west of it ; whereas in July 

 the lowest pressure, 29 -40 inches, is at a considerable distance from the above, being 

 located in the valley of the Indus and south-westwards to Muscat. The difference of 

 pressure between these two extreme months is thus fully 1'10 inch, or fully a thirtieth 

 part of the entire barometric pressure, nearly the whole of the difference being 

 occasioned by the difference of temperature of the two months. In North America the 

 difference of pressure of January and July is only 0*45 inch, and in Australia the differ- 

 ence is nearly the same. 



In the remarks on January it was pointed out that the centres of maximum pres- 

 sure and minimum temperature, which, Antarctic regions being excepted, are respectively 

 these maximum and minimum data for the globe for any season, are far from occupying 

 the same geographical area. But in July the regions of minimum pressure and maxi- 

 mum temperature are virtually coincident. In this region the climate is remarkably 

 dry and rainless, or nearly so, and substantially the same climatic characteristics 



