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



Australia, South Africa, and the greater part of South America. Generally speaking, 

 pressure has diminished where temperature has begun most markedly to rise, and the air 

 removed appears to be added to the portions of the atmosphere overspreading the 

 northern half of North America, Europe, Australia, and the region of the Atlantic 

 already referred to. None of the changes, however, are so material as to bring about 

 any serious difference in the prevailing winds as compared with those of January. 



March. — In March the lowest isothermal in Asia has now risen to —30°, and in 

 America to —25°, and over all the more strictly continental regions of the northern 

 hemisphere the great annual increase of temperature is rapidly proceeding ; but in the 

 more strictly insular and oceanic climates of the globe the change of temperature from 

 that of February is comparatively small, as is well shown by the isothermals of the 

 British Islands, Australia, and New Zealand. The marked increase of temperature on 

 advancing inland, from both the east and west coasts of the United States, and the 

 remarkable flexures of the isothermals of Europe and Asia, in the transition from winter 

 to summer, are very instructive. 



The great changes in the distribution of the pressure in this the first of the spring 

 months, are a large diminution overspreading the whole of Asia and Europe, except the 

 British Islands, the North Atlantic to the south of lat. 40°, and North America to the 

 south of lat. 50°. On the other hand, there occurs a very large increase of pressure to 

 northward of these Atlantic and American latitudes, amounting to upwards of a tenth 

 and a half in mid- Atlantic between the British Islands and Labrador ; and there is also 

 an increase, though less decided, over nearly the whole of the southern hemisphere, the 

 exception being the South Atlantic, lying between the increasing pressures of Africa and 

 America, which show rather a slight diniinution. 



In this month the extra-tropical waters of the oceans reach the annual extremes of 

 temperature, those of the North Atlantic falling to the annual minimum, and those of 

 the South Atlantic rising to the annual maximum. Now at this season this region of 

 the North Atlantic, lying between the rapidly-increasing temperature and falling pressure 

 of the Europeo- Asiatic and the American continents, receives an increment of pressure 

 much larger than takes place in any other month of the year. 



There are seven anticyclonic areas — in Central Asia, where pressure is rapidly 

 falling from its high winter maximum ; in British America, where it is rising to the 

 maximum in spring ; two in the Pacific and two in the Atlantic immediately to west- 

 ward of the continents ; and in the Indian Ocean west of Australia. The systems of 

 low pressure are in the north of the Atlantic and Pacific Oceans, in Central Africa and 

 round the South Pole. 



April. — This is the first month when the annual increase of temperature is largely 

 felt over both insular and continental regions. The increase is, however, larger in 

 continental climates, and particularly where the rainfall is comparatively small and the 



