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



of the North Atlantic to the south-east, the meteorological peculiarities which rendered 

 the weather of December 1878 memorable over nearly the whole globe arose out of 

 a distribution of the earth's atmosphere which was essentially the same that obtains 

 at this season, but the usual irregularities in the distribution of the pressure appeared 

 in more pronounced characters. 



Mean Atmospheric Temperature and Pressure for the Year. — The distribution 

 of the mean annual pressure may be regarded as representing the sum of the influences 

 at work, directly and indirectly, throughout the year in increasing and diminishing 

 atmospheric pressure and temperature. 



The isothermal of -5° surrounds the north pole, and marks off the region where 

 the annual temperature of the globe falls to the minimum, Maps XXV. and XXVI. The 

 regions of highest mean annual temperature marked off by the isothermal of 85° occur 

 in Central Africa, in India, the north of Australia, and Central America ; but, except 

 Central Africa, these areas are very restricted. Temperature is depressed in the greatest 

 degree towards the eastern sides of the land surfaces of the continents as they stretch 

 towards and into the Arctic regions. As regards the ocean, temperatures are low on 

 the eastern coasts of the continents of the northern hemisphere and on the western 

 side of the continents of the southern hemisphere. The effect of the more clouded 

 condition of the atmosphere of intertropical South America as compared with Central 

 Africa is well illustrated by the isotherm als of these two extensive regions. 



The most conspicuous example of the influence of ocean currents in raising the 

 temperature is seen in the protrusion northwards of the isothermals over western 

 Europe, due to the prevailing winds and widespread currents which there pass from 

 lower to higher latitudes. The contrast the temperature of the east coast of America 

 offers to that of Europe is very striking. A similar result, but in a greatly reduced 

 form, is seen on comparing the east of Asia with the west of North America. 



As respects land surfaces of tropical and sub-tropical countries, the highest mean 

 annual temperatures are found in those regions where for a considerable portion of the 

 year the climate is dry and practically rainless. The isothermals of Mexico and Brazil 

 show in a striking manner the ♦nfluence of dry and wet climates on the distribution 

 of temperature in low latitudes. In this connection the crowding together of the 

 isothermals in Africa and South America about latitude 30° S. is one of the most 

 striking features of these lines. 



The chart of mean annual atmospheric pressure shows two regions of high pressure, 

 the one north and the other south of the equator, which pass completely round the 

 globe as broad belts of high pressure. The belt of high pressure in the southern 

 hemisphere lies parallel to the equator, and is of tolerably uniform breadth throughout, 

 widening, however, in the longitudes of the anticyclonic regions of the Pacific, Atlantic, 

 and Indian Oceans, and of the less pronounced anticyclone of Australia, The belt of 



