CHAPTER XLVIII. 

 PHYSICAL GEOGRAPHY. METEOROLOGY. 



THE ATMOSPHERE WINDS AND AIR CURRENTS WIND PRESSURE 



STORMS RAIN - CLOUDS WATER-SPOUTS ATMOSPHERICAL PHE- 

 NOMENA. 



UNDER this heading we shall find the atmosphere playing a very im- 

 portant part. The air is composed of oxygen and nitrogen with some 

 carbonic acid gas and aqueous vapour. We have, under the Chemistry 

 section, discussed these constituents which unite to make up the air or 

 atmosphere in the following proportions : 



Oxygen. 210*0 



Nitrogen 775-0 



Aqueous Vapour ......... 14-3 



Carbonic Acid , o'8 



lOOO'O 



It is a fact that all over the world the same chemical result is found. 

 Whether we bottle up the air in the valley, or, as Gay-Lussac did, go up 

 to an elevation of 21,000 feet in a balloon, we shall find the air of the 

 same chemical composition. In Europe, Asia, Africa, and America, it is 

 all the same. The pressure is less as we ascend, and we cannot manage to 

 breathe in very high altitudes so well as upon the ground for which we were 

 fitted, but the air is the same. 



The atmosphere, then, is not always equal in density, nor is it quite 

 transparent. The light from sun and stars is, to a certain extent, lost, and 

 it has been calculated that the sun's rays lose one-fifth part of their bright- 

 ness passing through the atmosphere. We all know what the air is. We 

 breathe it, we feel it blowing, we witness its effects. Were it not composed 

 as it is we should die or go mad ; plants would not live, and the earth 

 would become a desert. Air is everywhere invisible ; a so-called empty 

 vessel is full of air because an animal will live in it till the atmosphere 

 has become vitiated by the carbonic acid from the lungs. Yet air, or rather 

 its watery portion, is visible when condensed. 



Vapour is not perceptible. But how does it become so ? We cannot 

 see the air, how can we see a portion of it ? We can answer this question 

 by illustration. The steam from an engine is not visible on a very hot 

 day. But when the day is damp and dull the vapour is condensed^ and 

 becomes visible ; then air appears and is resolved into vapour again. This 



