METHOD OF SHOWING TEMPERATURES 



135 



over a considerable region should be known and a record 



of it made and preserved. This might be done by taking 



a map and writing their 



temperatures above the 



different places marked 



on the map. This 



would make a map full 



of small figures and 



very difficult to read. 



A much better 

 method has been devel- 

 oped and is now almost 



HEAT BELTS. 



Notice how these heat belts vary from 

 the latitude zones shown on Figure 10, 

 page 25. 



universally used. In 



making this map the 



temperatures are first 



written on the map and then lines are drawn through 



places which have the same temperature. These lines are 



called isotherms and the map is called an isothermal map. 



By the use of such a map it is possible at a glance to de- 

 termine the temperature prevail- 

 ing at any place and to see the 

 relation which this has to the 

 temperature of other places on 

 the map. As a rule the isotherms 

 are not drawn for each degree, 

 but only for each ten degrees. 



When the map has been con- 

 structed, copies are made in which 

 the figures are left off and only 

 the isotherms are preserved. In 



Fig. 66. 



Figure 66 we have a plan before the isotherms are drawn, 

 and in Figure 67 after the isotherms are drawn. Figure 

 68 is a typical isothermal diagram. If the map itself were 

 sketched, it would be an isothermal map. 



