Spirit-level tubes are drawn out in a blowpipe flame; the tube, with care, does not cease to be a tube, though, when twisted hot, or broken cold and placed for a moment in the flame, it is quickly sealed up. These tubes are not quite straight, but the error is not great. A tube like Fig. 1 would be quite useless, as the bubble would divide into two portions, as shown, and no indication would be possible; but if the tube is turned over as in Fig. 2, the bubble promptly comes to the centre. The illustrations are purposely exaggerated. A tube that is quite straight for a portion of its length, and curves off towards one end, is very unsatisfactory; if the tubes could be uniformly larger towards the centre, as in Fig. 3, no care would be necessary in mounting. It is desirable that a tube, when set in place, should give similar indications when reversed, as in Fig. 3, although the surface is not level; this cannot happen unless the curvature of the tube is uniform, and the tube is uniformly set in its socket. Levels, as usually sold, are set on a tinfoil film, which makes the bubble more easily seen; in home-made levels a substitute may be employed. Mix dry plaster-of-Paris with a little powdered blue, or mix the plaster with water and blue ink; quickly set the convex side upwards, so that the bubble reverses equally at a slight inclination. An adjustable inclination is easily obtained by resting the tube on two screws inserted in the bench for a portion of their length. By this means, on reversing, the bubble ought to occupy similar positions as regards distance from the centre; then, after adjusting the screws until the bubble rests in the same place when reversed, the centre can be marked. Of course, long before this has been accomplished the plaster will have set; but this does not matter, as the under side can be adjusted by shaving with a sharp trying plane; treated thus, the level is more correct than if the tube were adjusted by the Angers. If the tube has been deeply embedded, the block containing it can be made parallel after the under surface has been adjusted.

Fig. I.

Fig. 2.

Fig. 3. Choosing and Fixing Spirit-level Tubes.