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 

 BO 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, $ ; 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 any 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 



