£44 MOISTURE OF THE AIE. 



as at its then present temperature it can take up, 

 is said to be saturated or fully charged with water- 

 vapour. It is said, for instance, to be half 

 charged if it holds fifty per cent, or three-quarters 

 charged if seventy-five per cent, of the greatest 

 amount of moisture that it can take up at its 

 actual temperature. The degree of charge always 

 shows, therefore, not the actual, but only the rela- 

 tive quantity of vapour held in the air. Thus in 

 the cold season of the year, when it contains but 

 a small actual amount of water, the air may be, 

 and in fact generally is, moister, or nearer to its 

 point of full charge, than in summer, when it 

 holds a far greater quantity of vapour. In the 

 latter case, indeed, evaporation may perhaps be 

 still briskly going on, while in the former it will 

 have almost ceased. Thus in the months of No- 

 vember and December the air generally seems to 

 us moister than on a warm day in July, when, 

 however, it will often contain two or three times 

 as much water. 



Over the great oceans the air is generally, at all 

 seasons of the year, and in all latitudes, near its 

 full-charge point. Over the land its condition as 

 to moisture depends essentially on that of the 

 ground, on the direction of the wind, and on the 

 distance from the sea. 



All bodies, without exception, have the power, 

 some more, some less, of taking from the air some 



