378 ANNUAL OF SCIENTIFIC DISCOVERY. 



eastern half, containing the largest mass of land, is colder than the 

 western, the difference diminishing constantly as the equator is ap- 

 proached. Within the tropics the diminution of temperature in going 

 northward is very regular. The average winter and summer tempera- 

 ture of the two hemispheres is shown by the following table : 



o 



January. Northern Hemisphere, .... 48.8 

 Southern " 59.5 



The Globe, ... . 54.15 



July. Northern Hemisphere, .... 71.0 

 Southern .... 53.6 



The Globe, 627s 



The temperature of the whole globe increases, therefore, fully 8 from 

 January to July. The mean temperature of the globe is 58. 2', of the 

 northern hemisphere 60, and of the southern 50. 4'. In accounting 

 for this, Prof. Dove says, "The earth's surface being a highly 

 varied one, the sun's influence upon it is also constantly varying, for 

 the impinging solar heat is employed in raising the temperature of 

 substances which do not change their condition of aggregation ; but 

 when engaged in causing the melting of ice or the evaporation of wa- 

 ter, it becomes latent. When, therefore, the sun returning from its 

 northern declination enters the southern signs, the increasing propor- 

 tion of liquid surface upon which it shines causes a correspondent part 

 of its heat to become latent, and hence arises the great periodical vari- 

 ation in the temperature of the whole globe which has been noticed." 

 We shall now briefly consider the monthly isothermal lines. Tak- 

 ing the temperature of 32, in January, the line drawn across the 

 globe passes from Philadelphia over the Banks of Newfoundland, 

 through the South of Iceland to the polar circle in the meridian of 

 Brussels. Hence in the direction of the meridian to Holland, thence 

 southeasterly to the Balkan ; from the middle of the Black Sea it 

 runs in a west and east course across Asia to the Ccrea, thence it 

 rises to the Aleutian Islands, and descends again in America to the 

 latitude of Palermo. In February the lines of equal temperature be- 

 gin to move northward in Northern Asia, while in North America 

 they are still moving southward. In March the spaces in America 

 and Africa inclosed by the isothermal of 8lA F. have united. In 

 April two spaces of unusually high temperature bounded by the iso- 

 thermals of 86 F. are developed in the middle of Northern Africa and 

 in the interior of Western India. Everywhere in Asia and Middle 

 Europe, the isothermals are almost parallel with the parallels of lati- 

 tude. The line of 32 F. still preserves its extraordinary bend, run- 

 ning from Cape Breton to the South of Greenland, through Iceland, 

 thence to the North Cape, and sinks on the crest of the Scandinavian 

 Alps down to the latitude of Drontheim, whence it bends eastward. 

 This phenomenon is caused by the drift ice in the Atlantic, the effect 

 of which is still more decided in May. From Nova Scotia to New- 

 foundland the isothermals are crowded more closely together. Mean- 



