METEOROLOGY. 127 



which are absorbed, amount of vegetation, character of the 

 soil, and state of agriculture. 



But among all these causes none have so important an 

 influence on determining the climate of a country as latitude 

 and altitude. The degrees of heat are not always equal for 

 the same latitude; thus at Rome, in latitude 63 north, the 

 mean temperature is the same as that of Raleigh, North 

 Carolina, in latitude 36 north. 



Lines passing through points on the surface of the earth at 

 which the mean annual temperature is the same, are called 

 isothermal lines. These lines do not pass round the earth in 

 a direct course like the parallels of latitude, but they vary so 

 as to assume a tortuous direction. 



The isochimenal lines, or lines of equal cold, or equal 

 winter, vary much more than the lines of equal summer. 

 The reason why latitude affects the temperature of a climate, 

 is because it varies the obliquity of the sun's rays in relation to 

 the earth. This, however, is not the cause of the difference in 

 the length of day and night at different places. 



The following table from Muller shows the length of the 

 longest day for the different latitudes. 



Polar Elevation. Length of longest day. 



12 hours. 



1644 / 13 



3048' 14 



4922' 16 



6323' 20 " 



6632' 24 " 



6723' 1 month. 



7339' 3 



90 6 " 



Altitude has an important effect on determining the mean 

 temperature on all places, whatever may be their latitude. 

 The temperature diminishes from the surface upwards as far 



