THE CUTTING EDGE. 123 



sary in the field, to keep a sharp cutting edge at all times 

 on the share. 



Note. It lias been shown in the Report of the Trial of Plows nt Utica, 

 that so far as yet determined by experiment in England, about thirty-five 

 pel' cent of the whole required draught is expended in overcoming the 

 friction of the implement on its bottom and sides, about fifty-five for 

 cutting the furrow-slice, and only about ten per cent for turning the 

 sod. Hence the exclusive attention formerly given to forming the 

 mould-board, as a means of reducing the draught, should have been 

 directed more to lessening the force Required for cutting the hard soil. 



These experiments, however, do not appear to have been entirely 

 satisfactory, especially for the light plows of this country ; and it may 

 be interesting to test their accuracy by calculation. The average weight 

 of hard earth is about 125 lbs. per cubic foot ; and the average draught 

 of plows at the trial near Albany- in 1850 was about 400 lbs. for a furrow- 

 slice a foot wide and six inches deep. If a team in turning such sod 

 moves two miles an hour, it raises a slice three feet long, equal to a 

 cubic foot and a half (weighing 187 lbs.,) six inches each second which 

 would be the same as raising 31 lbs. three feet per second, which is the 

 velocity of the plow. The mere force required to turn the sod, not esti- 

 mating friction, would therefore be only one-thirteenth of the 400 lbs. of 

 draught force. But the friction of dry earth on smooth iron is never 

 less than one-half its weight ; and if the earth is slightly plastic, its 

 friction often is equal to, and sometimes exceeds, its weight. Taking 

 the smallest amount, the friction on the mould-board would be equal to 

 half the weight of the portion of sod resting on the mould-board, or 

 about 31 lbs. This increased weight would also add equally to the fric- 

 tion of the sole of the plow, or 31 lbs. more making the whole friction 

 62 lbs. ; which added to the weight of the sod would amount to 93 lbs. 

 or more than one-fifth of the whole draught. 



To ascertain the amount of friction, suppose the plow weighs 100 lbs. 

 Half its weight would be 50 lbs., the friction on the sole of the plow. 

 The friction of the sides would vary greatly Avith plows, being very 

 small with those having a perfect centre-draught, or with no tendency 

 to press against the land on the left. The whole friction and force for 

 lifting the sod would therefore be about 150 lbs. ; leaving 250 lbs. as the 

 force for cutting the slice. A very easy running plow would leave a 

 much smaller force some as low as 200 lbs. 



This estimate is liable to great variation. A wet and clayey soil would 

 double the friction ; a very hard piece of ground would add much to 

 the force required for cutting the slice ; if loose, the force would be com- 

 paratively small ; or if quite moist, this force would be also much dira- 

 ished ; while the great difference in the draught of plows would vary the 

 results still farther. The estimate, however, for soil dry enough to be 

 friable, and of medium tenacity, is probably not far from correct, for 

 plowing in this country showing that most of the force required is for 



