874 



THE COMPLETE GRAZIER. 



BOOK IX. 



In calculating the cost of executing drainage, the following table, 

 given in the " Report of the Board of Health's Minutes of Information 

 on the Drainage of Buildings, Sites, and Roads," will be useful. The 

 cost of cutting drains obviously depends upon the quantity of earth 

 thrown up, and the nature of the material to be excavated. To ascer- 

 tain the cubic contents of a drain, multiply together the length, 

 depth, and mean width of the drain. The mean width is half of the 

 sum of the width of drain at the widest, and that at the narrowest 

 part ; thus, if the width at top is 18 inches, and width at bottom 4 

 inches, 11 inches, half of 18 -f- 4, is the mean width. The following 

 is the table above referred to : 



Along the top of the table is placed the mean widths in inches ; and 

 in the left-hand column are the depths of the drains, extending from 30 

 inches to 5 feet. The numbers in the body of the table express cubic 

 yards per rod. In making use of the table, it is necessary, first, to 

 find the mean width of the drain from the widths at the top and 

 bottom. 



Thus, if a drain 3 feet deep were 16 inches wide at the top and 

 4 inches wide at the bottom, the mean width would be half of 20 

 (=16+4), or 10; then, by looking in the table for the column under 

 10 (width), and opposite 36 (inches of depth), we find the number of 

 cubic yards in each rod of such a drain to be 1*53, or somewhat more 

 than l cubic yards. If we compare this with another drain 20 inches 

 wide at the top, 4 inches wide at the bottom, and 4| feet deep, we have 

 the mean width 12 ; and looking at the table under 12 and opposite 54, 

 we find 2'75 cubic yards, or two and three-quarters to the rod. In this 

 case the quantity of earth to be removed is nearly twice as much as in 

 the other, and hence, as regards the digging, the cost of the labour will 

 be nearly double. But in the case of deep drains, the cost increases 

 slightly for another reason, namely, the increased labour of lifting the 

 earth to the surface from a greater depth. 



The following table is introduced as showing the number of rods to 

 be laid, and the number of pipes required per acre, with drains at 

 various distances apart : 



