WTXD CLOUDS EAIX. 31 



column of mercury (which is many times heavier than 

 water) up to the height of about twenty-nine inches, 

 according to the weight of the air at any particular time ; 

 this weight varies according to the state of the weather, 

 whether wet or dry ; but the average w r eight of air is found 

 to be perfectly stationary, and during the twenty years from 

 1816 to 1836, it was found at Paris not to have varied y^Vo^ 1 

 of an inch. Winds or currents of air proceed from several 

 causes ; when the sun shines on a large surface of the earth, 

 it becomes heated and a column of hot air is pressed upwards, 

 for hot air is lighter than cold, and the cold air all around, 

 by its weight, forces it up and rushes in to fill the space ; 

 this space may perhaps be many hundred square miles in 

 extent, so that a current of wind is caused to blow towards 

 this spot from all the regions round. This is the general 

 state of things at the equatorial parts of the earth and causes 

 the " trade winds," \vhich uniformly blow (the greater part 

 of the year) from both north and south towards the equator. 

 Another cause of local winds, is the condensation of vapours 

 into water ; when the air over a large region is saturated 

 with moisture it is greatly expanded by it, and when the 

 vapour is condensed and falls as rain, the air from the 

 surrounding parts forces itself in to fill the space occupied 

 by the rain while in a state of vapour, which is nearly one 

 hundred thousand times greater than it occupies when in the 

 form of water. There are many causes to determine the con- 

 densation of the vapour, w r hich the atmosphere always holds 

 in larger or smaller quantities, but cold is the chief agent, 

 for when a current of air passes over seas or rivers, or the 

 damp surface of the earth, it becomes loaded with moisture, 

 and being capable of holding only a certain quantity (less 

 when cold than hot), it follows that warm air when saturated 

 with moisture must let some of it fall when it becomes 

 cooled. This may occur from entering a colder region, or 

 uniting with another current of air colder than itself. In 

 tropical regions the air is so warm that it takes up a very 

 large quantity of watery vapour and upon the coming of 

 colder weather, the rainy season begins and the air empties 

 itself of its superfluous moisture, causing the most tremen- 



