40 OP WATER. 



invisible eggs of microscopic animals, and thus cleanses 

 and sweetens the atmosphere. 



121. Evaporation from the surface of the earth always 

 cools it. But, on the condensation of ammonia, and the 

 other gases, the reverse must take place. The heat which 

 had held them in a gaseous form, is given to the water in 

 which they are absorbed and condensed, warms it, and, 

 sinking into the earth, warms the soil. 



122. Plants absorb a large quantity of water through 

 every part of their surface, but chiefly through their 

 roots. But by the action of light and heat, they exhale 

 a good deal of it through the leaves. You have only to 

 cover a plant exposed to the sun s light with a bell glass, 

 and you will presently see the inner surface of the glass 

 covered with dew, and soon after with little drops. The 

 evaporation which is going on from the surface of 

 leaves is one of the sources from which the moisture of 

 the atmosphere is supplied. As we are subject sometimes 

 to excessive heat and drought, and sometimes to excessive 

 rains, the object of the farmer should be to guard against 

 both, and to render his fields, as far as he can, indepen 

 dent of variations in moisture. 



123. We manage to prevent plants from suffering for 

 want of water by irrigation, that is, watering with little 

 streams, when these are possible and not too expensive ; 

 and by other artificial means. We can do something 

 towards it, often we can do a great deal, by keeping the 

 tops of the hills in our neighborhood covered with trees. 

 l h -so attract and impede the clouds, and induce them to 

 pour down their rain. 



1:M. DiM-p ploughing, by rendering the earth to a con- 

 oV|.ih r:ipahlo of retaining moisture, will also 



