Fig. 54.
"A is a common square, placed in a slit in the top of the stake B. By means of a plumb-line the square is brought to a level, when a thumbscrew, at C, fixes it fast. If the square is two feet long, and is so carefully adjusted as not to vary more than the twentieth of an inch from a true level, which is easily accomplished, then a twentieth of an inch in two feet will be one inch in forty feet—a sufficient degree of accuracy for many cases."
Fig. 55.—Square and Plumb-Level.
We do not so much object to the principle of the above level, as to its practical working. We find it difficult, without cross sights, to take an accurate level with any instrument. However, those who are used to rifle-shooting may hit tolerably near the mark with the square. Mr. Thomas only claims that it is accurate enough "for many cases."
A proper spirit-level, such as is used by engineers of railroads and canals, attached to a telescope, is the best of all instruments. "So great is the perfection of this instrument," says the writer just quoted, "that separate lines of levels have been run with it, for sixty miles, without varying two-thirds of an inch for the whole distance." A cheap and convenient spirit-level, for our purpose, is thus constructed.
It is furnished with eye sights, a b, and, when in use, is placed into a framing of brass which operates as a spring to adjust it to the level position, d, by the action of the large-headed brass screw, c. A stud is affixed to the framing, and pushed firmly into a gimlet-hole in the top of the short rod, which is pushed or driven into the ground at the spot from whence the level is desired to be ascertained. It need scarcely be mentioned, that the height of the eye sight, from the guard, is to be deducted from the height of observation, which quantity is easily obtained by having the rod marked off in inches and feet; but it may be mentioned, that this instrument should be used in all cases of draining on level ground, even when one is confident that he knows the fall of the ground; for the eye is a very deceitful monitor for informing you of the levelness of ground. It is so light as to admit of being carried in the pocket, whilst its rod may be used as a staff or cane.
