850.—The Author's Hand Level is shown Fig. 404—12 inch. This is made of a casting either of iron or brass. The level tube is ground to curvature and is somewhat superior to the ordinary run of this class of work. The level tube is fitted with ball socket at one end and stiff spring fitting at the other, which is adjustable, so that the tube may be easily replaced if broken.
These levels are commonly fixed upon a stout fir straight-edge of about 5 feet to 10 feet in length by the lugs at the ends. The level is taken by blockings upon the ground. Corrections of error, both in level and straight-edge, may be made for any considerable distance by reversing the forward and backward position of the level with its straight-edge alternately.
851.—Square Level—Circular Level.—Fig. 405 represents a very useful class of level for setting up some instrument stands, plane tables, etc., in which a pair of level tubes are placed at right angles to each other. It is generally made very small—1½ inches square only. A circular level, the upper surface of which is formed of a worked concave glass, was lately very popular, and is still used to a small extent. As the spirit cannot be hermetically sealed in, it evaporates, and this level soon fails. Mr. J. J. Hicks has taken out a patent for a hermetically sealed circular level, described [p. 96], which appears to answer very well.
Fig. 405.—Square level.
Fig. 406.—Surface level and clinometer.
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852.—Incline Level.—For laying railway rails and drainage works the bubble is frequently made adjustable by the tube in which it is contained being hinged at one end and fitted in slides to rise with a screw at the other end, as shown Fig. 406. A scale of percentage of inclination S is commonly divided upon the adjustable end. The tube is raised or lowered by the key A, which is removed after setting and cannot be tampered with.
Fig. 407.—Stanley's sight for mechanics' levels.