LEVELING THE DRAINS. 115 



be made as shown at fig. 52. It consists of a parallel- 

 edged board, seven or eight feet long, with a JL affixed 

 near one end, which supports a pendulum. A scale is 

 marked on the board at the foot of the pendulum, where- 

 by its motions are noted. "When the board is perfectly 

 level the foot of the pendulum marks 0. When the board 

 inclines either way it varies accordingly. A handle is 

 fixed to the end of the level, which serves to hold it in 

 position when in use. In case it is not wished to lay out 



Fig. 52. LEVEL. 



the bottom of a ditch to a very accurate grade, the mere 

 movement of the pendulum to the right, when looking at 

 the scale or index, will show that the grade is downwards. 

 But if accurate measurement is desired, it will be neces- 

 sary to make the instrument in proportion, and mark the 

 index carefully also with a proportionate scale. Thus, if 

 the bottom of the level is six feet long, and the JL two 

 feet high, an elevation of the hinder end of the instru- 

 ment of half an inch would be equal to a grade of one 

 inch in 12 feet, or one in 144, or eight inches in 100 feet, 

 and would cause a deviation from the perpendicular of 

 the pendulum of one-sixth of an inch ; a grade of 16 

 inches in 100 feet would cause a deviation of one-third 

 of an inch. If such close measurement is desired, the 

 instrument will have to be carefully made. For ordinary 

 operations, it will only be necessary to take care that the 

 JL is set on quite square, and then the least movement 



