56 THE VOYAGE OF H.M.S. CHALLENGER. 



season has not yet begun, or where the rainfall is not large. Of this, India, Central 

 Africa north of lat. 10° N, and the more strictly inland regions of North America from 

 about latitude 15° N. in a northerly direction, are the best examples; and in a less 

 degree the more continental portions of the Spanish and Scandinavian peninsulas. 

 The contrast in this respect of India and the Eastern Peninsula is very striking, the 

 relatively low temperature of the latter being probably due to the " lie " of its great 

 valleys in the line of the summer monsoon. The influence of the Red Sea and Persian 

 Gulf in this and subsequent summer months in breaking the continuity of the 

 isothermals and changing their course is very remarkable. The low temperature of 

 the north-eastern portion of America and over the north of Siberia as compared with 

 Western Europe is probably occasioned by the northerly winds which have now set in 

 towards the rapidly developing centres of low pressure in the interior of the continents 

 taken in connection with the sun's position in the heavens. 



Accompanying these changes of temperature, pressure has fallen greatly over 

 nearly the whole of the continents of the northern hemisphere, the amount of the fall 

 being generally the same as in the previous month ; and again the fall over the Arabian 

 Sea and Persian Gulf is only a half, or even less, than in India, lying between these two 

 seas. A diminution of pressure has also taken place over the south-east of Australia, 

 New Zealand, the southern portion of South America, and over the sea immediately to 

 the south of Cape Colony. 



On the other hand, pressure has continued further to increase over nearly the 

 whole of South America and Africa. But the region of the great increase of pressure, 

 or the region to which has been transferred the mass of the earth's atmosphere which 

 has been removed from the Asiatic and American continents, is the Atlantic Ocean 

 from the Arctic Circle south and to at least lat. 20° S., exclusive of the Caribbean Sea, 

 but inclusive of the United States east of the Mississippi and Ohio, Lower Canada, and 

 nearly the half of Europe, to the south of a line drawn from Shetland to the Sea of 

 Azov. The maximum increase, being nearly two-tenths of an inch, occurs in mid- 

 Atlantic, about lat. 45° N. In the Atlantic, from lat. 55° to 70° N, pressure now 

 attains its annual maximum. 



A high pressure overspreads nearly the whole of Arctic regions, the maximum, 

 30*10 inches, extending from the mouth of Back Eiver to Nova Zembla. The other 

 anticyclonic areas of high pressure, in addition to the four in the Pacific and Atlantic 

 Oceans, are found in the centre of South America, in South Africa, to the south of 

 Madagascar, and in Australia. Of these the least pronounced is the one in Australia, 

 and that most pronounced is in the Pacific to the west of California, where* pressures 

 are respectively 30 '05 and 30 '30 inches. Pressure has increased over the an ticy clonk- 

 region of the North Atlantic ; and as pressure all round has considerably fallen, tins 

 anticyclone is now a strongly marked one. 



