244 PHYSIOGRAPHY 



of temperature between January and July is much greater in 

 high latitudes than in low. Thus in the southern part of Hudson 

 Bay there is 70 difference between January and July; at Lake 

 Erie, about 45; in Florida, about 20; and near the equator in 

 South America, less than 10. The same charts show that the 

 difference is greater in the interiors of continents than on coasts 

 or over the sea in the same latitude. Thus in the interior of North 

 America, west of Hudson Bay, the difference is about 80, while 

 on the coast of Alaska it is only about 30. 



The courses of the isotherms. 1 . The courses of the isotherms 

 are, in a general way, east-west ; that is, they are roughly parallel 

 to the parallels of latitude. Some of them are very irregular, it 

 is true, but the east-west direction is the most common one. This 

 shows some relation between the courses of isotherms and latitude; 

 but since the isotherms do not follow the parallels exactly, it is clear 

 that latitude is not the only thing which determines their position. 



2. Figs. 217 and 218 show that the isotherms are straight- 

 est where there is little land, and most crooked where there is 

 much land. This suggests that the land and water have some- 

 thing to do with their positions. There are various irregularities 

 in the isotherms on land that do not appear on the sea. Thus, 

 on the January chart there is an area in south Africa, and another 

 in Australia, surrounded by the isotherm of 90, and there are 

 similar areas in North America, northern Africa, and southern 

 Asia, in July. All of these areas are on land. These facts tend to 

 confirm the conclusion that the sea and the land influence the posi- 

 tion of the isotherms. 



Following this idea still further, it is seen that the isotherms 

 of January frequently bend somewhat abruptly in passing from 

 water to land, or from land to water. Thus the isotherm of 30 

 in the northern hemisphere turns to the south when it reaches 

 North America, and again on the coast of Europe. In the south- 

 ern hemisphere, the isotherms of 80 and 70 make abrupt turns 

 at the west coast of Africa, and the isotherm of 70 near the west 

 coast of South America. These bends at the coasts give further 

 support to the conclusion that the distribution of land and water 

 have something to do with the position of isotherms. 



