76 FLORA INDICA. 



being expanded and made lighter, it immediately ascends, its 

 place being supplied by air from colder regions. Thus, since 

 no two places have the same temperatm'e, and since the tem- 

 perature constantly changes, even in the same place, the at- 

 mosphere is kept in constant motion. 



As the amount of aqueous vapour which is capable of re- 

 maining suspended in the atmosphere is directly proportional 

 to the temperature, ascending currents of air finally become 

 so cooled that condensation or precipitation takes place ; and 

 the nearer to saturation the air is before it begins to ascend, 

 the sooner it wUl reach a sufficiently low temperature for 

 condensation. We can therefore understand why mountain- 

 chains (which impede the direct course of the currents, and 

 force them to ascend) cause precipitation of the moisture of 

 an atmosphere which has already traversed, withoiit any con- 

 densation, a great extent of level country. 



The direction of the wind is primarily dependent upon the 

 sun's position, and is a very complex phenomenon, in conse- 

 quence of the perfect fluidity of the air. On the open sea, at 

 a sufficient distance from land to escape its influences, the 

 trade-winds, owing to the intertropical heat, blow with great 

 regularity towards the equatoi', or rather towards a point im- 

 mediately under the sun's position, varying therefore with the 

 season of the year. Their direction is not due north and 

 south, but more or less towards the west. This is in conse- 

 quence of their retaining the momentum proper to the lati- 

 tude whence they start, in their advance towards the equa- 

 tor, where the motion of a point on the earth's surface (due 

 to its revolution round its own axis) is a maximum. They 

 therefore lag behind, as it were, and appear to blow from 

 the north-east in the northern hemisphere, and from the 

 south-east in the southern hemisphere. The presence of 

 land interferes with the regularity of the trade- winds; and 

 '.vhere it occurs in large masses, it becomes so much more 

 heated than the ocean, that it attracts the aerial currents to- 

 wards itself, and hence completely changes the direction of 

 the wind. 



