For scientific purposes a millimetre run per second is commonly used, then—
arc sec (206264·8) × ·001 metre = 206·264 metres radius or 680 feet nearly.
For an ordinary 12-inch dumpy level the tube is divided into minutes at about 1/10 inch apart, radius 28 to 30 feet; for a sensitive 14-inch Y-level of good construction the same divisions may represent five seconds, radius of bubble tube about 170 feet.
175.—The Divisions upon Ordinary Level Tubes are made after the tube is finished, but with the highly sensitive ones the divisions are made first. The run is taken from ten to thirty divisions on each side of the centre of the tube, where it is lightly marked with a marking diamond. These spaces are then equally subdivided and etched in with hydrofluoric acid or marked with a hard steel edge dipped in turps. If further refinement be required, the errors of run in relation to the divisions are tabulated from the testing of the tube with the bubble trier. A less careful method is employed by some makers of leaving the level tube undivided and fixing an independent metal or ivory scale over it.
176.—Level tubes are generally filled with pure alcohol for ordinary purposes; for trade purposes with methylic alcohol, which is much cheaper. For very delicate tubes sulphuric ether or chloroform is used. The sensitiveness of the bubble depends very greatly upon the mobility of the liquid with which it is filled, and to the quality of adhesion of the liquid to the glass. The relative mobility of the above-mentioned liquids is found by delicate tests with the bubble trier for small distances under the microscope at a temperature of 60° Fahr. Taking water as 100:—we find commercial methylic alcohol 22, absolute alcohol 13, sulphuric ether 5, chloroform 3,—that is, for equal small runs taken in 15 seconds of time. All bubbles appear to be more or less affected by temperature, particularly where the spirit is not nearly absolute. In the higher temperatures the bubbles are more active. The objection to chloroform, where it is likely to be subject to great changes of temperature, and where there is no provision made for regulating the size of the bubble—the means of doing which will be presently discussed—is that its expansion from heat is so great that it is very liable to burst its tube. It can therefore only be used with ordinary sealed tubes where these are small and strong. Sulphuric ether has the same fault, but in a lesser degree.
177.—The sealing of ground tubes requires the skill of a very experienced glass-blower, and is a technical matter on which no written instructions would be of value under any conditions. A little strain is unavoidably put upon the tube in sealing with the blow-pipe, so that the curvature to which it is worked is more or less disturbed. For this reason level tubes which are required to be of the highest degree of accuracy are sometimes left as they are ground, and closed at the ends by small discs of glass grooved to the end surfaces. These are fixed on with glue, and when the glue is set are bound over with silk and finally varnished; but this plan is much too delicate for instruments for use in the field.
Fig. 48.—Colonel Strange's level tube.
Larger image
178.—Colonel Strange's Level Tube.—These tubes, Fig. 48, are blown with an outward bead at each end of the tube, two outwardly screwed collars, F, being first placed over the tube before the blowing. The tube is then ground to curvature. A plug, S, is formed for each end of the tube from a plano-convex lens, ground to a bevel on the plano side, and also ground into the end of the tube as a stopper. A cap, C, is screwed over the end upon the collar. The springiness of the cap keeps the stopper always tight. As there is no blow-pipe used after the grinding, the tube remains constant as it is ground, or it can be adjusted by grinding to any desired sensitiveness. This cap, for security, is better covered with silk tied over it, and afterwards well varnished. In this class of tube there is always a little risk of evaporation. The system is not adapted to instruments to be used in the open air.