104 



HANDY-BOOK OP HUSBANDRY, 



marked on the stakes, and, for the distance at which the rate of 

 fall is unchanged, the line of tiles should be a straight line, lying 

 at the computed distance below those stakes. 



The computation may be made as shown below. For illus- 

 tration we take the first three spaces of the main drain (J) on 

 our map (fig. 30). 



Note that the Jirst column represents the marks on the stakes ; 

 the second is the measured distance between the stakes ; the third 

 the total descent of one hundred feet of drain, (eight-tenths of one 

 foot and six-tenths of one foot per one hundred feet ;) the fourth, 

 the amount of fall, at this rate, from stake to stake ; the sixth, the 

 recorded grade at each stake. The first figure of the fifth repre- 

 sents the recorded grade of the floor of the outlet, (21.71,) and by 

 subtracting from this the fall between y/ and Ja, (.34,) we ob- 

 tain the grade of the drain at the latter stake, (21.37 ;) subtracting 

 from this the next fall, (.36,) we have the grade at Ab, etc. ; then, 

 by subtracting the figures in the sixth from those in the fifth, we 

 have in the seventh the depth of cutting at each stake. 



FORM OF COMPUTATION FOR DEPTH OF DRAIN BELOW TOPS OF GRADE STAKES. 



For want of space, in the map on page loi, only such points 

 have been marked with letters and grades as are necessary to illus- 

 trate the text. 



The least rate of fall which it is prudent to give to a drain, in 

 using ordinary tile, is 2.5 feet in 1,000 feet, or 3 inches in 100 feet, 

 and even this requires very careful work.* A fall of 6 inches in 



*Some of the drains in the Central Parle have a fall of only one inch in one hun- 

 dred feet, and they work perfectly; but they are large mains, laid with an amount of care 

 and with certain costly precautions, (including very precisely graded wooden floors,) 

 which could hardly be expected in private work. 



