142 MECHANICS. 



vigilance on the part of the attendant ; and this imiform- 

 ity, by preventing uneven running, lessens the aggregate 

 amount of draught. It is, however, quite important that 

 the wheel sustain little or no pressure ; for as soon as the 

 beam bears upon it, the line of draught becomes crooked 

 at the expense of the team. These facts were established 

 by careful expeiiments with the dynamometer. 



PULVEEIZEKS. 



The fine pulverization of the earth, for the ready ex- 

 tension of the roots of plants, for the action of air on the 

 soil, for the retention of moisture, and for the thorough in- 

 termixture of manure, is of great importance to the farmer. 

 It is but partially accomplished by the plow, which crum- 

 bles the soil only so far as may be done by the act of 

 turning it over. Henee additional implements are needed 

 for this purpose, among which arc the harrow, the cultwa- 

 tor, and the clod-crusher. 



HARROWS. 



The Brush-hcirroio, the original and rudest form of the 

 implement, and still used for covering grass seed, as often 

 made, is a poor implement. The 

 most projecting limbs are cut partly 

 off, that all may lie flat, but it often 

 happens that the projecting angles of 

 ^the larger branches plow into the 



ground and make deep furrows. This 

 Brush-harrow. , , t i r. i -i 



may be prevented by a careful selec- 

 tion of the small tree which forms the brush, or still better 

 by constructing a simple rough plank frame, so that any 

 quantity of short brush may be placed between two pieces 

 of plank, to admit the tops of the brush to incline down- 

 wards and backwards, being lield in place by a few spikes 

 or bolts. Fijr. 149. 



