This is necessary on outdoor structures which are to be placed on
foundations, in securing level or horizontal timbers, and in plumbing
the uprights. The human eye is not equal to the task. Masons and
builders make use of wooden plumb rods, but as the level is necessary
to secure the horizontals, it will be at hand for the uprights, the two
glass tubes being at right angles. (Fig. 131.)
[Illustration: Fig. 131. The spirit level]