332 PRACTICAL LESSONS IN SCIENCE. 



son the relative extent of land and water areas and the relative 

 positions of the bodies of land and water are important climatic 

 questions. 



At first it seems as if there might have been more land ; but 

 on examination we find that only a portion of the present land 

 surface can get water enough to be productive of plant or animal 

 life. The land extends in two masses almost from pole to pole, 

 and with such general form and irregular outline that very large 

 portions come under the equalizing influence of the sea. 



The land varies in elevation. There are low plains, plateaus 

 and mountains. The important facts to be learned about moun- 

 tains are their location, elevation and direction of extent ; and in 

 regard to plateaus their location, elevation and extent should be 

 known. In general the mountain systems of the Western conti- 

 nent trend north and south, and so do those of South Africa and 

 Australia, while in Europe and Asia the trend is more nearly 

 east and west. The greater mountain systems border closely on 

 the deeper oceans, as the Pacific and Indian. 



The air is an invisible gas surrounding the earth, extending 

 upward some 400 or 500 miles. It is composed mainly of oxygen 

 and nitrogen, with varying quantities of water vapor, carbon 

 dioxide, ammonia, nitric acid, etc. Variation in the temperature 

 of the air is the principal cause of winds, rainfall, dew, frost and 

 other phenomena, which together make up what we call climate, 

 and which is generally discussed under the heads of temperature, 

 winds and rainfall. 



The temperature of the air depends mainly on the temperature 

 of the earth's surface. The heat from the sun passes through the 

 air and warms the earth, and the earth in turn warms the air 

 mainly by convection, partly by radiation and reflection. It is 

 estimated that the air absorbs more than one-third of the heat 

 sent from the sun to the earth, the greater part being taken up 

 by the water-vapor and carbon dioxide, as pure air is nearly dia- 

 thermic. The air absorbs even a larger per cent, of the heat radi- 

 ated from the earth and prevents the surface from cooling as 

 rapidly as it would if the air were diathermic. 



