CHAPTER IV.
LEVELS—METHODS OF ASCERTAINING—LEVEL TUBES—MANUFACTURE—CURVATURE—SENSITIVENESS-TESTING—READING—CIRCULAR LEVELS—SURVEYORS' LEVELS—Y-LEVEL—PARALLEL PLATES—ADJUSTMENTS OF Y-LEVELS—SUGGESTED IMPROVEMENTS—DUMPY LEVEL—TRIPOD STANDS—ADJUSTMENT OF DUMPY—COLLIMATOR—IMPROVEMENTS IN DUMPY LEVEL—TRIBRACH HEAD—DIAPHRAGMS—CUSHING'S LEVEL—COOKE'S LEVEL—CHEAP FORMS OF LEVEL—HAND LEVELS—TELESCOPIC LEVEL—REFLECTING LEVELS—WATER LEVELS.
166.—A Level Plane is understood technically to be a plane truly tangential to the theoretical spheroidal surface of the earth, as represented by any spot upon the mean surface of the ocean or of still water free from local attraction. The importance of having the means of constructing efficient instruments that can be conveniently employed to obtain the correct relative altitudes of points or stations upon the earth's surface, in relation to such a plane or datum, can scarcely be overrated. Such instruments are not only used for topographical surveys of countries, but also in designing and carrying out public works adapted to the local conditions of natural inclination of the land surface, for railways, drainage, irrigation, canals, water-works, and other constructions.
167.—The force constantly at our command to enable us to ascertain relative altitudes and to form mentally or graphically local level lines on the earth, is that of gravity; and it is only a question in any case how the action of this force shall be employed. There are four principles which we may accept as data for employing gravity, each depending upon a natural phenomenon:—(1) The open upper surface of a liquid unaffected by currents of air, or the influence of solid objects in close proximity causing capillary action, or local attraction of solid masses, represents a level plane. (2) The line of a plummet unaffected by currents or lateral attractions forms a vertical line to which the level plane is everywhere at right angles. (3) The atmospheric pressure, from the approximated equality of its density due to its weight in proportion to its height over limited areas, gives pressure according to its gravity—therefore altitude or difference of level relatively to lesser pressure compared with a lower datum. This pressure is measurable with a barometer or other form of pressure gauge. (4) The resistance to ebullition in a liquid is inversely proportional to the weight or pressure of the aërial fluid resting upon its surface. This is measurable by the temperature at which liquids boil under varying atmospheric pressures. Various instrumental refinements have been discovered to render these natural phenomena available in practical use for ascertaining difference of height. The first and most exact method employed for this purpose, by means of the liquid plane, will be considered in this chapter. The other methods will be deferred to later pages.
168.—In taking the level of a liquid surface contained in a vessel, we have, as just stated, to keep this surface free from the disturbing influence of air currents, and to surround the surface with equal conditions of capillary attraction, or to make these conditions equal in the direction in which we desire to ascertain our level. This is found practically to be best performed by means of a sealed glass tube, in which the liquid will by gravitation naturally occupy the lower place, and any air or lighter fluid contained therein the space above this.
Fig. 47.—Level tube (bubble).
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169.—Level Tubes, or Bubble Tubes, as they are technically termed, are used as a part of nearly all important surveying instruments. One of these is represented Fig. 47. The glass for the construction of these tubes is drawn at the glass-houses in lengths of about 6 feet, and may be ordered of any desired size and substance. The tubes are drawn of as nearly straight and equal bore as possible. They are, nevertheless, found to be, when examined after annealing, curved more or less in various directions at different parts of their lengths. They are found also generally to be slightly tapering from end to end and of slightly unequal substance. In the manufacture of level tubes parts of the tube are selected with approximately regular longitudinal curvature, and these parts are cut off into the required lengths by a triangular file dipped in spirits of turpentine, to be ready for the future operations of grinding, sealing, and dividing. After the tube is cut off and carefully examined to get its most concave internal surface upwards, this is then marked by a test mark, with the flat of a file, near one end for future work and reference. The grinding of the inside of the glass tube to true curvature is performed by passing it over a brass mandrel or core, which is employed to grind the glass by means of fine emery. The core is turned slightly barrel-form to the longitudinal curvature intended for the upper surface of the finished tube. It is made of full three-quarters the diameter of the interior of the tube, and a little longer than the entire tube. This core is attached by its ends to two stiff but flexible wires of brass, about 8 B.W.G. for a tube of ·7 inch diameter, and these wires are held firmly by their ends in two vices, so that the core is slung, as it were, to permit a certain amount of flexibility under the pressure of the hand used in grinding. Some good makers do not use a mandrel core, but only a strip of brass on the mandrel, extending about 60° of the circumference. In this case the strip has to be corrected for curvature during the grinding, which plan is sometimes preferred for certainty. The grinding of a tube cannot be commenced with coarse emery, such as is used in the grinding of lenses, as the cut of a coarse emery will quickly split the tube. After the glaze is removed there is not so much risk, so that a little time may be saved by passing a current of hydrofluoric acid gas through the tube; but more careful testing is required afterwards, as the cut of the grinding tool is not so evident at sight when the glazed surface is removed.