179.—Chambered Level Tubes.—As the run of a bubble varies slightly with its size, for exact purposes and extreme climates it is very desirable to be able to adjust the size of the bubble to the surrounding temperature, so that it shall be kept at about equal dimensions for all measurements made with it. This becomes particularly important where chloroform is used, from the expansion being very great. A general way of doing this is to have a stopper ground into one end of the tube, which is itself a small bottle, on the under side of which a hole is ground, so that by turning the tube over and raising it more or less, any amount of the highly rarefied air it contains may be taken to form the bubble that may be desired. The stopper is fixed with thin glue. The general construction is shown below, Fig. 49. Of course where such a tube is used there must be means of tipping and turning the instrument in which it is fixed over, or the bubble itself must have separate fixings. The portability of a surveyor's level admits readily of the necessary tipping; with theodolite levels at right angles to each other upon the vernier plate it would be impossible.
Fig. 49.—Bubble with supplemental air-chamber.
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Fig. 50.—Colonel Scott's patent protected bubble.
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180.—Extra Strong Level Tubes.—Colonel Scott's very ingenious device of enclosing a level tube within another tube of thoroughly annealed glass will be found valuable in all cases where the tube is much exposed, or where it is difficult afterwards to procure a new tube in the case of accident. These tubes are at present only made by the author for Scott's telescopic gun-sights, which are nearly like small theodolites. The level tube, Fig. 50, is made as stout as possible to be soundly sealed after filling. It is then enclosed in an annealed tube, CC′, of about ·08 to ·12 inch in thickness, the interspace between the two tubes being filled with Canada balsam. It is then plugged with elastic marine glue, KK′ and cemented over PP′. The annealed tube is of great strength, so that the complete naked tube thus formed will bear dropping on the ground, and also when attached to a large gun will bear the violent vibration of firing without injury.
Fig. 51.—Artificial horizon level.