592 MR ALEX. BUCHAN ON THE MEAN PRESSURE OF THE ATMOSPHERE. 



from the S.E. and S., may be expected to be warm and moist, it may further be 

 inferred, that the winter climate, so far as regards the two important elements of 

 heat and moisture, is subject to great fluctuation. On the other hand, since the 

 July mean temperature is about 80°, and in the same month the wind blows ten 

 days from S.E. and fourteen from S., it may be concluded that the summer 

 climate will be hot, stifling, and relaxing. These are shown, by observation, to be 

 the peculiar characteristics of the climate of Shanghai in winter and in summer. 



4. An inquiry of still greater interest is suggested by the isobaric lines of the 

 Charts. Their position appears to be altogether determined by the geographical 

 distribution of land and water on the surface of the earth ; and since the isobaric 

 lines determine the prevailing winds, and these in their turn the peculiar distri- 

 bution of temperature and rainfall,— in other words, the climates of the globe, — 

 it is evident that we have here a principle applicable not merely to the present 

 state of the earth, but also to different distributions of land and water in past 

 time. In other words, there is here a principle which the geologist will require 

 to apply in attempting to account for glacial and warm epochs, through which 

 the climates of great Britain and other countries have passed. In this way it 

 is possible to arrive at an approximate numerical statement, as regards tempera- 

 ture and rainfall, of Sir Charles Lyell's idea of the changes of climate brought 

 about through the displacements of continents. 



The following instances will serve to illustrate the effect of the partial dis- 

 placements of continents in changing climate. On examining the chart for July 

 (Plate XXVI.), it is seen that the fine summer climates of Western Europe, and of 

 the Eastern States of America, are caused by south-westerly prevailing winds, 

 which, having their origin in the region of high pressure in the Atlantic, possess, 

 in admirable proportions, the genial qualities of warmth and moisture. Since these 

 winds depend on the high pressure in the Atlantic between Africa and the United 

 States, whatever would alter this arrangement of the pressure may be expected to 

 change the character of the climates. Suppose, then, a displacement of the con- 

 tinents, either of Africa or South America, so that land would occupy the place of 

 the part of the ocean lying between Africa and the United States. With this new 

 disposition of the land, it is plain that the high pressure in the Atlantic would 

 disappear, and the spaces of low pressure in Asia, Africa, and North America 

 would unite into one region of low pressure, stretching from the west of North 

 America to the east of Asia. Simultaneously with this change in the pressure, 

 the winds of the United States and Western Europe, including Great Britain, 

 would become northerly, and, as a consequence, the summer climates of large 

 portions of these regions would be so seriously deteriorated that the cultivation 

 of cereals could not be attempted. 



Observations show that the lowest pressures which accompany the storms 

 which traverse Europe, or the centres of these storms, pass eastward for the mosl 



