VI.] COOLING BY EVAPORATION 109 



at the surface is joined up to that surrounding the 

 particles in the subsoil below, with the result that as fast 

 as water is evaporated from the surface the losses are 

 repaired from the subsoil. This not only causes a loss 

 of water to the soil — a loss which may become of 

 importance later — but the continuous evaporation keeps 

 the surface moist and cold. It should always be realised 

 that an evaporating liquid withdraws heat from the 

 objects with which it is in contact — the heat being 

 necessary to convert the liquid into vapour. This with- 

 drawal of heat is well seen in the cold experienced by 

 the wetted hands or face when a dry east wind is 

 blowing and causing rapid evaporation; it may be 

 illustrated experimentally by wrapping the bulb of a 

 thermometer in wet flannel or cotton-wool and whirling 

 it rapidly round and round in the air, whereupon the 

 temperature indicated by the thermometer may be 

 made to fall several degrees. Again, the cooling effect 

 of evaporation may be demonstrated by the cold 

 experienced when a little alcohol, ether, or petrol is 

 poured on the hands. Being all readily volatile liquids, 

 they withdraw a considerable amount of heat from the 

 hands in order to pass into the state of vapour. The 

 spring cultivation of the previously ploughed land 

 breaks up the continuity of the water films between the 

 plough layer and the subsoil below, water thereupon 

 travels much more slowly into the surface layer, which 

 in consequence can become dry, and being dry can also 

 warm up at an earlier date. The actual drying also 

 will often take place a little more gently, because the 

 land is not forced to wait until the parching east winds 

 set in ; when heavy wet land is dried rapidly under 

 their influence a hard, steely crust is generally formed 

 on the surface which is difficult to break down later. 

 The early spring ploughing before the frosts are quite 



