REPORT ON ATMOSPHERIC CIRCULATION. 75 



On the other hand, the least difference of the lowest and the highest mean 

 monthly pressures, 0"05 inch, occurs in four isolated patches. These regions, indicat- 

 ing thus the greatest stability of mean pressure throughout the year, occur all in 

 equatorial regions, viz. in the East Indian Archipelago; in the Pacific, from 150° to 

 105° long. W. ; and again from 95° to 75° long. "W. ; and in the Atlantic to the west of 

 Senegambia, extending only from about 10° of longitude and 6° of latitude. These are 

 all included in a wide area, almost wholly restricted to intertropical regions, bounded 

 by 0"10 inch, and stretching unbroken from the east coast of equatorial Africa east- 

 wards across the whole of the Pacific, the north of South America, the Atlantic, and 

 into Africa, as far as about 5° long. W. Other isolated patches, showing also the small 

 difference of O'lO inch, occur to the south-west of Australia, South Africa, and South 

 America ; in the South Pacific between 40 a and 50° lat. S. and 130° and 180° long. W. ; 

 in the North Pacific to west and south-west of California ; to the east of Sagalien and 

 Japan ; and in the Gulf of Bothnia and Finland. 



This, perhaps, of all the annual phenomena disclosed by meteorology, presents the 

 strongest contrast between the northern and southern hemispheres. The northern 

 hemisphere, with its large masses of land, shows the maximum variability in the mean 

 pressure through the months of the year. Indeed, in extropical regions the difference 

 does not fall so low as O'lO inch except in three insignificant patches. In the southern 

 hemisphere, with its enormous breadths of ocean, the range shows comparatively small 

 variability. It is only by the low pressures of the winter months, when temperature 

 and humidity of the air over the northern portions of the Atlantic and Pacific Oceans 

 are abnormally high, that the ocean may be regarded as contributing to the formation 

 of a range of as much as 0"40 inch to the mean monthly pressures of the year. 



The Isobaric Maps show, in the clearest and most conclusive manner, that the 

 distribution of the pressure of the earth's atmosphere is determined by the geographical 

 distribution of land and water in their relations to the varying heat of the sun through 

 the months of the year ; and since the relative pressure determines the direction and 

 force of the prevailing winds, and these in their turn the temperature, moisture, 

 rainfall, and in a very great degree the surface currents of the ocean, it is evident that 

 there is here a principle applicable not merely to the present state of the earth, but also 

 to different distributions of land and water in past times. In truth, it is only by the 

 aid of this principle that any rational attempt, based on causes having a purely 

 terrestrial origin, can be made in explanation of those glacial and warm geological 

 epochs through which the climates of Great Britain and other countries have passed. 

 Hence the geologist must familiarize himself with the nature of those climatic changes, 

 which necessarily result from different distributions of land and water, especially those 

 changes which influence most powerfully the life of the globe. 



