230 



The Span or Rafter Level. 



Vol. VI, 



ment will be found very useful in difjg-ing 

 cellars, laying out the foundations of build- 

 ings, and a variety of other purposes that will 

 readily suggest themselves to every farmer. 

 It differs in form, but not in principle, from 

 the common level board of the carpenter. 

 Its only advantage is in combining greater 

 strength and stiffness with less weight of 

 materials. To fit it for the purpose of giving 

 a slope to a drain, or for any similar purpose, 

 place it so that the plummet-line shall coin- 

 cide with the mark on the cross-piece, then 

 place a block, say an inch thick, under one 

 of the feet, and mark the place the line 

 crosses the cross-piece ; then add another 

 inch under the same foot, and mark again, 

 repeating the operation as often as may be 

 thought necessary. Now take out the blocks 

 and go over the process again, by placing 

 them one at a time under the contrary foot, 

 and making the necessary marks on the cross- 

 piece. The lines should then be numbered, 

 1, 2, 3, &.C., from the middle one. It must 

 be evident from the construction of the in- 

 strument, that when it is so placed that the 

 plummet-line falls on mark No. 1, or that 

 nearest the middle one, that the end of the 

 instrument which is then nearest the plummet- 

 line will be one inch lower than the other — 

 when on No. 2, two inches, &c. And also, 

 in using it, as drains are usually cut in the 

 ascending direction, the plummet-line must 

 be made to fall behind the middle mark, to 

 give them the required rise or slope. The 

 instrument, if marked on both sides of the 

 middle line, can be used either end foremost. 

 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 No. 1 

 will give a fall, or rise, of one inch to the 

 perch, or 26 feet 8 inches to the mile — when 

 resting on No. 2, two inches to the perch, 

 &c. And if the span be made 12 feet 4^ 

 inches, or three-quarters of a perch, and the 

 blocks used for raising the feet in marking be 

 made three-quarters of an inch thick, each 

 of the lines will show the same fall as above. 

 Fractional parts of an inch may be found by 

 dividing and subdividing the spaces between 

 the lines, or by using blocks of the proper 

 thickness. 



The instrument as above divided will serve 

 to show the fall in certain proportions to a 

 given length, but there are many 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 feet 8 inches, 13 feet 4 inches, 

 or 26 feet 8 inches to the mile, and sundry 

 others, by various additions of them together. 

 Now, if the fall of the ground were found to 

 be any number of feet and inches between 

 these, it could not be properly proportioned , 

 in the drains by the use of the above lines. | 



A ready method to obviate this difficulty, and 

 fit the instrument to meet every case, would 

 be to make the span of it 12 feet 6 inche."?, or 

 the one-eighth part of 100 feet; then ascer- 

 tain the number of inches of fail there is 

 in 100 feet (in the directions of the drain) 

 of the ground to be drained, and for every 

 inch thus found, let one-eighth of an inch be 

 allowed to the block used in raising the feet 

 of the instrument when making the slope 

 marks; or let the block be as many eighths 

 of an inch thick as there are inches of fall 

 in 100 feet. Thus, if the fall in 100 feet be 

 6 inches, the block must be six-eighths or 

 three-quarters of an inch thick ; 7 inches, 

 seven-eighths; 8 inches, 1 inch, &.c. In such 

 cases, the lines on the instrument (except the 

 middle ones) should be made with a lead- 

 pencil, or red chalk, so as to be readily ef- 

 faced when they are done with, and it be- 

 comes necessary to make others. By making 

 the instrument of this length, which we be- 

 lieve will be found more convenient in using 

 than any other, it can by the above method 

 be at any time, in a few minutes, adapted to 

 any case that can occur. — The principle upon 

 which the whole rests being simply to find 

 the fall of a portion of the ground to be 

 drained, or drain to be made, equal in length 

 to the span of the mstrument, and to apply a 

 block in making the marks equal in thickness 

 to that fall. But to ascertain this fall, free 

 from the errors occasioned by small undula- 

 tions in the ground, considerable lengths 

 must be taken, say not less than 100 feet, 

 which must afterwards be divided into such 

 lengths as will mnke a portable instrument. 

 For finding the difference in level between 

 distant points, the span level is not the most 

 appropriate instrument. Some remarks I 

 have to make on this branch of the subject I 

 must defer to another occasion. 



In giving the foregoing description of the 

 Span Level, and the method of adapting it 

 to be used as a slope instrument, it is not 

 supposed that the subject is exhausted, or 

 that other forms and methods might not be 

 pointed out ; the object being to explain and 

 illustrate in as plain and familiar a manner 

 as possible, the principles on which the in- 

 strument is constructed, and the method of 

 applying those principles to practice. For 

 this purpose, methods and measures plain 

 and easily comprehended by all have been 

 used at the risk of being thought tedious and 

 common-place. The subject is a very plain 

 and simple one in itself, yet there are a vast 

 number who are not familiar with it; and 

 that there are many others who but imper- 

 fectly understand it, we need not go further ; 

 than the pages of the Cabinet to determine. 

 It is for the benefit of these, and not for the 

 wise or learned, that I am now writing. 

 January 22, 1842. S. LeWI8. 



