358 TEMPEBATURE OF THE EARTH MODIFIED BY VARIOUS CAUSES. 



was much higher than at present. In addition to this, we have in our 

 strata, the fossil bones of enormous amphibious reptiles, and the shells 

 of marine animals like the nautili, that exist at present in equatorial 

 seas : we thus obtain an accumulation of evidence, both from the 

 water as well as the land, in proof of the same position. The pres- 

 ent temperature of the earth appears to be dependent on two causes, — 

 the radiation of heat from the sun, and internal fire. That the tem- 

 perature of different latitudes is in a considerable degree dependent 

 on solar radiation will not be disputed : it increases with the increase 

 of the sun's meridian altitude as we advance towards the equator, 

 and it increases and decreases in the same latitude, with the increase 

 and decrease of the sun's altitude in different seasons. The tem- 

 perature of different countries in the same parallels of latitude is 

 very much modified by various causes : between the tropics, at the 

 height of about fifteen thousand feet, we meet with eternal snow. 

 In the Swiss and Savoy Alps, the line of perpetual congelation is 

 about seven or eight thousand feet : yet in the canton of the Vallais, 

 the upper valley of the Rhine, surrounded by snow-clad mountains, 

 is subjected to an oppressive heat in the summer months. Thus, 

 difference of elevation has, in all latitudes, a certain effect on tem- 

 perature. 



Large elevated continents in high latitudes, greatly decrease the 

 temperature of the air, by presenting a great surface of snow and 

 ice to the atmosphere. On the contrary, near the equator, large 

 continents raise the temperature greatly, by the constant radiation of 

 heat from the ground. The ocean, in different seasons, preserves a 

 more uniform temperature than the lajid ; hence, islands surrounded 

 by large seas, possess a more equal temperature throughout the year, 

 than continents in the same latitudes. The lines of equal tempera- 

 ture (called isothermal lines) are not parallel to the lines of latitudes, 

 as they would be, were temperature not affected by the causes before 

 stated. Quebec, with its Siberian winter, is nearly in the same par- 

 allel of latitude as Rochelle, in France, and is not two degrees north 

 of the latitude of Bourdeaux; a difference not greater than between 

 London and Nottingham, which in this country produces scarcely a 

 perceptible effect on the climate. In some countries, the summer 

 temperature is much greater than that of other countries in the 

 same parallel of latitude, yet the average annual heat, or what is 

 called the mean temperature, as measured by the thermometer, is 

 the same in both ; because, though the summers may be hotter, the 

 winters are proportionally colder, which reduces the average temper- 

 ature to an equality. But though the mean temperature may be the 

 same, the greater periodical increase and decrease of temperature in 

 one country than in the other, occasion a considerable difference in 

 the vegetation. If, on a good map, we examine, in the same parallel 

 of latitude, two situations which possess very different degrees of 

 temperature, we may generally observe a variation in the relative j 



