"3. Examine the staff regularly before setting it down to see that no dirt is sticking to the bottom of it.
"4. Always stand erect behind the staff, so that the figures face the level.
"5. Do not let any part of the hand come before the face of the staff.
"6. In no case put a downward pressure on the staff.
"7. If the grass be long, mossy, or spongy, tread it down, so that the staff shall have a firm footing—select a firm spot if the selection is left to yourself."[11]
285.—The manner of setting up a level has been already described in the previous chapter. The leveller generally follows a definite track which he has previously arranged and marked out on a map. The distances apart for placing the staves or staff are measured by the chain, or by the subtense system to be fully described hereafter. Where the levelling is very important, as for canal work, topographical survey, etc., wooden pegs are driven down at the measured stations where the staff is to be placed from which the levels are to be taken. A general rule followed, as far as practicable, for starting is to select an easily recognised, permanent, solid station for first placing of the staff—a mile-stone, large boulder, or other solid object answers: a datum line is generally assumed to be at a certain depth below this, to which all levels are referred. From this station, if the ground be fairly level, 5 chains is the ordinary advanced position where the level is set up and the first staff reading taken. The level is set up at the measured distance from the staff, which is indicated by a mark left by the chainman.
Fig. 111.—Level height tape.
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286.—Occasionally in town surveys the height of the level has to be taken. For this a small steel spring pocket tape is used to take the height of the axis of the telescope, Fig. 111. The tape may be adjusted by taking a piece off the first end, and allowing for the width of the tape case, so that by placing the ring of the tape upon the hook under the instrument and bringing the case just to the ground, the height of the axis of the telescope above the ground may be read off at the point where the tape leaves its case.