134 MECHANICS. 



discovered, than by ascertaining what we wish to know 

 only by long-repeated trials. 



TRENCH AND SUBSOIL PLOWING. 



When the common two-horse plow alone is used by 

 farmers, it pulverizes the soil only a few inches in 

 depth, and its own weight, and the tread of the horses 

 on the bottom of the furrow, gradually form a hard 

 crust at that depth, through which the roots of plants 

 and the moisture of rains do not easily penetrate. 

 Hence the roots have only a few inches of good soil on 

 the surface of the earth for their support and nourish- 

 ment ; and when heavy rains fall, the shallow bed of 

 mellow earth is soaked and injured by surplus water. 

 Again, in time of drought, this shallow bed of moist- 

 ure is soon evaporated, and the plants suffer in conse- 

 quence. 



But, on the other hand, when the soil is made deep, 

 it absorbs, Uke a sponge, all the rains that fall, and 

 gradually gives off the moisture as it is wanted dur- 

 ing hot and dry seasons. For this reason, deep soils 

 are not so easily injured by excessive wetness, or by ex- 

 treme drought, as shallow ones. In addition to this 

 advantage, they allow a deeper range for the roots in 

 search of nourishment. 



Soils are deepened by trench-plowing and by suh- 

 soiling. By trench-plowing, the common plow with 

 a mould-board is made to enter the earth to an unu- 

 sual depth, and to throw up a portion of the subsoil, 

 covering with it the top-soil which is thrown under. 

 A subsoil plow, on the contrary, only loosens the sub- 

 soil, but does not lift it to the surface. 



