200 HOW RAIN IS FORMED. 



giveu to inetisure the amouut lost by evaporatiou. lu England it has 

 been tbuud to vary in dififerent years from 17 to 27 inches iu the year, 

 or to say from 1| to 2^ inches per month on an average. Now, since 

 iu the east of England the rain-fall is only about 24 iuches iu the year, 

 it follows that in that part of the Kingdom the loss by evaporation 

 from a water surface is not very much less than the rain falling directly 

 on the surface. 



In dry countries the evaporation may exceed the local rain-fall. In 

 the tropics it has been found to average from 3.| to iuches per month 

 in the dry season. In the case of a large tank at Nagpur, constructed 

 to supply the city with water, it was found that the loss by evaporation 

 in the hottest and dry est weather was two and a half times as great 

 as the tiuautity supplied for consumption. 



These statistics will give some idea of the enormous evaporation that 

 goes oufrom the water surfaces of the globe, and to this must be added 

 all that takes place from the land. In the case of light showers nearly 

 the whole of the rain is re evaporated, and probably on an average half 

 of the total raiu-fall on the laud is thus lost sooner or later, leaving not 

 more than half for the supply of springs and rivers. 



The quantity of vapor in the air is very variable. To us, in England, 

 the west and southwest winds are the dampest, coming direct from the 

 x\tlautic, and northeast winds are the driest. The cause of their ex- 

 treme dryness I shall endeavor to explain presently. It is no doubt 

 partly due to the fact that they reach us from the land surface of Europe, 

 but partly also to another cause to which I shall have to advert later on. 

 The quantity of vapor in the air is usually ascertained by the hygrom- 

 eter, the ordinary form of which is a pair of thermometers, one hav- 

 ing the bulb wet, the other dry, and observing the depression of the 

 wet bulb. The principle of this I have already explained. But the 

 same thing may be ascertained more directly by passing a measured 

 quantity of air through a light apparatus containing sulphuric acid, or 

 some other substance that absorbs water vapor greedily, and weigh- 

 ing the whole before and afterwards. The increase of the second 

 weighment gives the weight of water absorbed. By such means it has 

 been ascertained that air at 60° can contain as much as of grains of 

 vapor in each cubic foot, and that air at 80° can contain rather less 

 than 11 grains in the same space. The quantity that air can hold in- 

 creases therefore very rapidly with the temperature. But it is seldom 

 that.it contains this maximum amount, especially at the higher tem- 

 I)eratures. 



In order to condense any part of this vapor we must take away its 

 latent heat. It is not sufficient merely to cool it till it reaches the tem- 

 perature of condensation, but we have further to abstract 5^ times as 

 much heat as v.ould raise the condensed water from the freezing to the 

 boiling point. Before however proceeding to consider how this cool- 

 ing is effected, the (piestion arises, W.hat is the condensing point ! For 



