No. 105.] 229 



in the dead furrows. This is shown by the section represented by 

 fig. 3, c c c c^ being the distributing channels, and d d d, the drains. 



Laying out the channels. — This can never be done with suffi- 

 cient accuracy without a levelling instrument. The cheapest for this 

 purpose is represented by fig. 4. It consists of the two legs AB and 

 ACj which should be light, but not less than ten feet long, connected 

 and stiffened by the cross bar, the two ends of which must be exactly 

 equidistant from the point A. A plumb line is suspended from the 

 summit A, and if the legs stand on level ground, the line will cut 

 the cross bar exactly in the middle ; but if the point of one leg be 

 lower than the other, the line will fall out of the centre and towards 

 that lower leg. The difference in the level of the two legs will be 

 accurately indicated by the graduated scale on the cross bar. If, for 

 instance, the distance AD, be one-third the length of one of the 

 legs, (the angle formed by the two legs being a right angle,) then a 

 movement of the plumb line half an inch from the centre, would 

 indicate a ditference in the level of the two points of the legs of 

 about two inches. The following is a general rule for this purpose 

 applicable to all cases with sufficient accuracy, where the ground does 

 vary greatly from a level. Multiply the distance AD, by the num- 

 ber of times the leg exceeds it in length, then, as fourteen is to 

 twenty, so is the product to the difference in the level of the two 

 points of the legs. 



If the two points are therefore one rod apart, a channel may be 

 expeditiously and accurately laid out, so as to have a slope of two 

 inches in a rod, by placing the level in such a position that the plum 

 will fall half an inch from the centre if the distance AD is one-third 

 of AB, or seven-tenths of an inch from the centre, if AD is half 

 of AB, according to the above rule. Successive points are thus 

 found by alternately bringing forward the legs of the instrument, 

 each being two inches lower than the preceding. Small sticks are 

 driven into the ground at these points, and by them a uniformly de- 

 scending furrow is easily and accurately plowed for a distributing 

 channel. If half that slope only is needed, one-half the distance 

 from the centre of the graduated bar is to be taken. 



The preceding course of marking the channel is more particularly 

 adapted to uneven ground, where every rod of distance must be ac- 

 curately known. But on large tracts of nearly level land, it will be 



