134 The Realm of Nature CHAP. 



North America approach at one place on the same map to 

 within one-tenth of an inch of each other, so that between 

 them one-hundredth of an inch corresponds to a change of 

 i. The space between the isotherms is coloured to bring 

 out the difference of temperature, the hottest regions being 

 shown in deepest red, the coldest in deepest blue. Isotherms 

 are constructed to refer to sea-level, so in order to find from 

 the maps the actual temperature at any place a deduction 

 of i for every 270 (or for convenience say 300) feet must 

 be made ( 163). For every place on the contour-line of 

 600 feet of elevation 2 must be deducted, and for every 

 place on the 6000 foot contour-lines 20 must be deducted 

 from the isothermal temperatures. Those two contour lines 

 are marked on the maps as a guide to the interpretation of 

 the results. 



The maps in this volume are reduced from the most 

 recent set of isotherms, compiled in connection with the 

 scientific reports of the Voyage of the Challenger^ by Dr. 

 A. Buchan, and they give the average temperatures for the 

 fifteen years from 1870 to 1884. 



189. Air Temperature in January. January is the 

 midsummer of the southern hemisphere. The map (Plate 

 III.) shows that the region with a temperature over 70 lies 

 south of the Tropic of Cancer on land, and the only places 

 warmer than 90 are under the Tropic of Capricorn in 

 Africa and Australia, the land being more heated than the 

 water by the nearly vertical Sun. The eastern sides of the 

 southern continents are warmer than the western ; thus on 

 the Tropic of Capricorn, the east coasts of Africa and South 

 America have a temperature of 80, and the west coasts less 

 than 70. This is explained by the prevailing winds and 

 ocean currents ( 243). The isotherm of 32 in the southern 

 hemisphere occurs about 64 S., and its direction is nearly 

 east and west, being uninfluenced by any land. Farther 

 north, the direction of the isotherms becomes more ir- 

 regular on account of the increasing interference of land 

 in altering the temperature of the air. In the northern 

 hemisphere, where January is midwinter, the sea as a rule 

 is warmer than the land in the same latitude, and the 



