230 PHYSIOGRAPHY 



by the length of day (hours of sunshine) and the direction of the 

 sun's rays. For the year, most heat is received in latitude 0. 

 A square mile in latitude 40 receives about three-fourths as much 

 as an equal area at the equator, and a square mile about the poles, 

 about half as much. During the half of the year when the sun's 

 rays are vertical north of the equator, March 21st to September 

 22d, most heat is received in latitude 25 N. Between May 31st 

 and July 16th the North Pole receives more heat than any other 

 part of the earth, the continuous day more than offsetting the great 

 obliquity of the sun's rays at this time (Fig. 207). 



The temperature of one place is not necessarily higher than 

 that of another because it receives more heat. No amount of 

 heat, for example, would make Greenland warm until after the snow 

 and ice were melted. The region about the North Pole does not 

 get very warm, even when it receives more heat than the equator, 

 because much of the heat received is expended in melting ice and 

 in warming ice-cold water, which is warmed very slowly, and runs 

 away as soon as the heating is well begun. 



Secondary distribution of heat. After the heat from the sun 

 has been received by the earth, it is redistributed to some extent, 

 with the general result that the parts which get more by insolation 

 share their heat with the parts which get less. 



There are three ways in which the air receives, loses, and trans- 

 fers heat. These are radiation, conduction, and convection. 



1. Radiation. When the sun shines, it radiates heat, and the 

 surface which its rays strike is warmed by absorption of the radiated 

 heat. A body need not be glowing hot, like the sun, or like fire, 

 to radiate heat. Hot water, steam, etc., radiate heat, as in the 

 radiators in our houses. The body which radiates heat is itself 

 cooled. Thus a hot piece of iron soon cools in the air, because it 

 radia,tes its heat. The land warmed by the absorption of heat 

 radiated from the sun during the day, is cooled by the radiation 

 of its heat during the night. The rate at which a body loses heat 

 by radiation depends upon the difference of temperature between 

 it and its surroundings. A hot stove will cool more quickly in a 

 cold room than in a warm one. 



2. Conduction. If one end of an iron poker is put in the fire, 



