Cultivation and tillage 



become too cold. If there is frost during the winter 

 both farmer and gardener are pleased because they say 

 the frost " mellows " the ground ; you can see what they 

 mean if you walk on a frosty morning over a ploughed 

 field. The large clods of earth are no longer sticky, 

 they already show signs of breaking up, and if they are 

 not frozen too hard can easily be shattered by a kick. 

 The change has been brought about in exactly the same 



^ 



Fig. 40. 



After liarvest the farmer breaks up his land with a plough and then 



leaves it alone until seed time 



way as the bursting of water-pipes by frost. When 

 water freezes it expands with enormous force and 

 burets open anything that confines it; water freezing 

 in the pores of the soil forces the little fragments apart 

 This action is so important that further illustrations 

 should be looked for. A piece of wet chalk left out on 

 a frosty night often crumbles to pieces. It is dangerous 



6-2 



