132 



MECHANICS. 



all mould-lDoards, care must be taken that the soil is 

 not lifted so abruptly as to throw it forward, instead of 

 simply turning it over. 



Another defect in some plows is too blunt or thick a 

 wedge formed by the share and mould- 

 board. By the plow represented in 

 Fig. 106, the earth must be thrown 

 from the land-side into the furrow 

 with a velocity about equal to the 

 motion of the team; but by the 

 one shown by Fig. 107, the team 

 moves twice as fast as the earth is 

 thrown by this longer wedge. C on- 

 sequently, according to the rule of virtual velocities 

 (aheady explained), as applied to the wedge, there is 

 a great gain in power. Care must be taken, how- 

 ever, not to make this wedge too long, else the friction 

 of a greater length of sod may overbalance the advant- 

 age. 



An attention to such principles as these has result- 

 ed in an extraordinary improvement within the past 

 thirty years. Plows are made with one third the for- 

 mer cost, that will do more than twice as much work 

 with the same strength of team, and do it so much 

 better, that larger crops may be reaped from the same 

 land. These advantages are so great, that on all the 

 arable land of the Union there must be a yearly sav- 

 ing of ten millions of dollars in the work of teams, one 

 million in the price of plows, and millions of bushels 

 in the aggregate increase of crops by good tillage. In 

 the two annexed figures we have a representation of 

 an old and an improved plow. 



