138 



THE SPAN LEVEL, FOR DRAINING. 



To fit the instrument for giving descent to a 

 drain, place it so that the plummet or bob-line shall 

 fall into the mark on the brace-piece ; then place a 

 block an inch thick under one of the feet, and mark 

 the place where the line crosses the brace ; then 

 add another inch block, and mark again, repeating the operation as 

 often as it is wished ; then take out the blocks, and go over the 

 process again, by placing them one at a time under the contrary foot, 

 making the necessary marks on the brace ; the lines should then be 

 numbered 1, 2, 3, &c. from the middle. Now, when the Level is 

 so placed that the bob-line falls on mark No. 1, or that nearest the 

 middle line, it is evident that the end of the instrument nearest the 

 bob-line will be one inch lower than the other ; and when on No. 2, 

 two inches, &c. And if the span of the feet be made 16 feet 6 

 inches, or one perch, and the lines marked as above, the plummet- 

 line, resting on mark No. 1, shows a fall or rise of one inch to the 

 perch, which is 26 feet 8 inches to the mile ; when resting on mark 

 No. 2, two inches to the perch, &c. And if the span be 12 feet 4 

 inches, or three-quarters of a perch, the blocks used for raising in 

 marking must be three-quarters of an inch thick, then each of the 

 lines will show the same fall as above. The Level as above divided, 

 will show the fall in certain proportions to a given length ; but there 

 are cases in draining where these relative proportions do not exist on 

 the ground : thus, the above lines and their subdivisions would show 

 a rise or fall of 6 ft. 8 in. 13 ft. 4 in. or 26 ft. 8 in. to the mile, and 

 sundry others, by various additions of them together; but if the fall 

 of the ground were found to be any number of feet and inches be 

 tween these, it could not be properly proportioned in the drains by 

 the use of the above lines ; but a ready way to obviate this difficulty, 

 is to make the span 12 feet 6 inches, the 8th part of an 100 feet ; 

 then ascertain the number of inches of fall in 100 feet of the ground 

 to be drained, and for every inch let one-eighth of an inch be allowed 

 to the block used in raising the feet when making the slope marks ; 

 or let the block be as many eighths of an inch thick as there are 

 inches fall in 100 feet thus, if the fall in 100 feet be 6 inches, the block 

 must be 6-8 or of an inch ; if 7 inches, i ; 8 inches, 1 inch, &c. 

 An instrument of this length is more convenient than any other, and 

 can be, by the above method, at any time adapted to &ny case that 

 might occur ; the principle being simply to find the fail of a portion of 

 the ground to be drained equal in length to the span of the instrument, 

 and apply a block in making the marks equal in thickness to that folk 



