28 BODIES OF WATER. 



amount of its specific heat. While the land changes 

 its temperature very considerably with the diurnal 

 and annual alternations, the sea does so to a less 

 degree. During the night, the efi'ect of water upon 

 the coast is to warm, and in the day to cool the 

 land. The same results are produced on the change 

 of the seasons : both the spring and autumn are 

 later. Places on the shore of large bodies of water 

 have frequently a clouded sky, and these arrest the 

 rays of heat from above, as well as prevent its loss 

 by radiation from beneath. Therefore, island or 

 coast climates are never subject to such extremes of 

 weather as are those of the interior. M. Boussin- 

 gault states that the temperature of London and 

 Paris is about the same, while the latitude of the 

 latter is nearly three degrees lower. 



As we travel from the coast toward the interior, 

 the difference between the mean temperature of 

 summer and winter is much greater than on the 

 seaboard. Rivers, lakes, and ponds possess to some 

 extent the same power of equalizing temperature, 

 according to their size ; and therefore we can per- 

 ceive the great value of isothermal lines established 

 by Baron Von Humboldt, which are a guide to the 

 general temperature, independent of the parallels of 

 latitude. 



By reference to an isothermal map which gives 

 the mean temperature of the United States for 

 the four seasons, we shall ascertain that this mean 



