CLiMATK. 153 



much aft'ects the climate, and as a general rule the western sides 

 of continents and large islands, are warmer than the eastern. Cer<- 

 tain portions of the globe, which from their nearness to the equa- 

 tor would be extremely warm, are rendered tolerably cool by their 

 elevated situations. This is the case with much of the tropical 

 land hi America, which is so raised that it rivals even European 

 climates in mildness and agreeable temperature. The air of these 

 elevated tropical districts is remarkably pure and transparent, and 

 the winds which sweep over the plains, are cooled by their pass- 

 age down the snow-capped mountains, which rear their bright 

 summits to the skies. The vast expanse of table-land, forming 

 the empire of Mexico is of this character, being elevated 7000 

 feet above the level of the ocean. This land in many parts has 

 the fertility of a cultivated garden. The plains of Columbia in 

 South America, and indeed all along the ridge of the Andes, are 

 similarly situated. The chart which we have given represents 

 the direction of .the isothermal lines, or lines connecting places 

 which have the same mean annual heat. It will be evident that 

 places may thus be situated on the same isothermal line, which 

 have very unequal mean temperatures of summer and winter. 

 We need only refer to the table on page 157, to be convinced of 

 this. Thus, the mean annual temperature of London, and Cam- 

 bridge, Mass, is the same, 5036'; but the mean temperature o 

 the warmest month at London is 6440', while at Cambridge it 

 is 7286', and of the coldest month, at London 3776, at Cam- 

 bridge 29.84, London therefore has a colder summer and a warm- 

 er winter than Cambridge. The reason of this, is undoubtedly, 

 the insular situation of the former, for as a general rule the ex- 

 tremes of temperature are experienced in large inland tracts, and 

 little felt in islands remote from continents. The difference be- 

 tween the mean temperature of summer and winter is nothing at 

 the equator, and increases continually with the latitude. When 

 the mean annual temperature is low the differences between the 

 extremes of the seasons is great, and the contrary. 



The effect of climate upon the geographical distribution of 

 plants and animals is very marked. Each, generally has its pe- 



