Drive a nail in a smooth even wall, to set one end on. Adjust the other end until the bulb is centered on the glass, and draw a line the length of the level. Reverse the opposite end of the level on the nail, center the bulb and draw another line. If the lines are parallel, the level is true; if they divide, it is not true. Take 1/2 the distance of the divided lines and draw a line from this point to the intersection of the lines for the level line, and adjust the level to the center line, and the level will be true. (Fig. 70.)

Fig. 70

2nd. Set it on an even surface and adjust the level until the bulb centers, reverse the level end to end, if the bulb centers, the level is true; if not, adjust the level until the bulb will come to the same place in reversing the level. (Fig. 70.)

Fig. 71