74 



THE VOYAGE OF H.M.S. CHALLENGER. 



large rainfall, the western half of it is singularly dry and practically rainless, and its 

 central portion occupies a region where at the time the climate is one of the driest and 

 hottest found at any season anywhere on the globe. Hence, while observation shows 

 the vapour to be the most important and widespread of the disturbing influences at 

 work in the atmosphere, the temperature also plays no inconspicuous part directly in 

 destroying the equilibrium of the atmosphere ; from which disturbance result winds, 

 storms, and many other atmospheric changes. 



Annual Range of the Mean Monthly Pressure.— This has been calculated from 

 the sea-level pressures by simply subtracting the lowest mean monthly pressure from 

 the highest, and entering the difference in its place on a map of the globe from which 

 the lines of equal difference were drawn, as shown in the accompanying Fig. 3. 



Fig. 3. Chart sbowing the annual range of the mean monthly pressure over the globe, expressed in hundredths of an inch. 



The greatest difference occurs in the interior of Asia, near Urga, to the south- 

 south-west of Lake Baikal, amounting to one inch. Thus in this region a thirtieth 

 part of the whole winter pressure is removed during the summer months. In British 

 America the difference is about - 40 inch, and this is also the difference in South 

 Africa. In South America and in Central Australia it amounts to 0"30 inch. These 

 all occur in continents, and the largest difference is in the largest continent. On the 

 other hand, in the North Atlantic, between Iceland and the south of Greenland, and 

 again in the North Pacific to the south of Alaska, the difference is 0"40 inch ; but in no 

 other part of the ocean is there so large a difference. In these two cases it is wholly 

 due to the exceptionally low winter pressure of these regions. In the southern 

 hemisphere the two patches of greatest difference occur, one to the east of New Zealand, 

 between 140° and 160° long. W., and the other in the Indian Ocean, to the south-west 

 of Australia, from 80° to 115° long. E. 



