Page 90 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.40 THE WORK

FIGURE 2.36B Using a hand level.

It may also be used as an inclinometer by placing it on the slope to be measured, and setting
the spirit level until the bubble is centered. It is usually good practice to lay a board on the ground
surface and take its slope to eliminate the effect of small irregularities.
String Level. A string level is convenient to use over short distances. It is a spirit level fitted with
prongs by which it can be hung from a string stretched tightly between two marks. Elevations may be
taken from the end of the string, or by measuring down from any part of it, as illustrated in Fig. 2.37.
The string used should be strong enough to take sufficient pull to remove all sag. If it is at all
slack, it will give false readings, showing slope at the ends of a level stretch and level somewhere
near the middle of an inclined string. A special need is depicted in Fig. 2.38.
Most string levels use flexible prongs which are easily bent by light pressure. It is therefore best to
check such a level before every use, and occasionally during a job. This is done by leveling a string
according to it, slacking the string and reversing the direction of the level on it, and tightening the string
to the same marks. If the reading is the same, the level is all right. If it disagrees, bend a prong sufficiently
to move the bubble one-quarter of the way to the center, then move the string to center the bubble.
Reverse the level on the string, and repeat the procedure as above, until the reading is the same both ways.