158 PROBLEMS OF BTRD LIFE. 



southward streamer that straggles south almost to Mexico ? 

 Why is the red-breasted nuthatch never seen in summer south 

 of the northern tier of states except along a narrow line in the 

 East and another in the West reaching hundred of miles south 

 of his natural home ? In answering these questions we shall 

 show that distribution is principally a matter of temperature, 

 or of temperature and moisture, which is climate. 



Did you ever notice in your geographies (but not all geog- 

 raphies have them) little, fine, brown lines that wave about 

 over the map like a filmy cobweb, now looping downward, 

 now hooping up, but in general travelling east and west? 

 They are called isothermal lines, that is, lines of equal heat, 

 because all the places through which they pass have the 

 same average temperature. There are also isotherms for 

 every month in the year passing through all the places that 

 have the same heat in summer and the same degree of cold in 

 winter, but these are not put down on our maps. 



In a general way, we were aware that it is warm in the 

 South, and that it grows colder as we go north, but perhaps 

 it is new to us that the change is not uniform ; in other words, 

 that the isotherms do not run straight across the map parallel 

 with the degrees of latitude. It is much more interesting as 

 it is. The isothermal lines now tell us considerable about 

 the country they cross, so that if all the lakes, rivers, and 

 mountains were removed from the map and only these lines of 

 average temperature left, we still might know something of 

 the surface of the country, while a map of the summer iso- 

 therms would tell us a great deal. 



When an isotherm takes a northward bend, we know that 

 the heat is greater inside the loop than it is outside it. 

 Usually we find within the loop either a flat plain that re- 

 flects the heat, or a lower level of land along some river 



