Thus it happens that as the bell descends into the water past the lower angle of _S_, it begins to require more space for itself in the tank, and so it displaces the water until the levels rise. When high, as shown in the sketch marked A^3(a), the water-level is at _l_, below the mouth of a pipe _P_; but when low, as in A^3(b), the water is raised to the point _l'_, which is above _P_. Water therefore flows into _P_, whence it reaches the carbide in an attached decomposing chamber. Here also the water in the tank is used for decomposition as well as for sealing purposes, and its normal level must be maintained exactly at _l_, lest the mouth of _P_ should not be covered whenever the bell falls. [Illustration: FIG. 1.--TYPICAL METHODS OF AUTOMATIC GENERATION CONTROLLED BY BELL GASHOLDER.] The devices employed to render a generator automatic which depend upon pressure (B) are of three main varieties: (B^1) the water-level in the decomposing chamber may be depressed by the pressure therein until its surface falls below a stationary mass of carbide; (B^2) the level in a water-store tank may be depressed until it falls below the mouth of a pipe leading to the carbide vessel; (B^3) the current of water passing down a pipe to the decomposing chamber may be interrupted by the action of a pressure superior to the force of gravitation. These arrangements are indicated roughly in Fig. 2. In B^1, D is a hollow cylinder closed at all points except at the cock G and the hole E, which are always below the level of the water in the annulus F, the latter being open to the air at its top. D is rigidly fastened to the outer vessel F so that it cannot move vertically, and the carbide cage is rigidly fastened to D. Normally the water-levels are at _l_, and the liquid has access to the carbide through perforations in the basket. Acetylene is thus produced; but if G is shut, the gas is unable to escape, and so it presses downwards upon the water until the liquid falls in D to the dotted line _l"_, rising in F to the dotted line _l'_. The carbide is then out of water, and except for after-generation, evolution of gas ceases. On opening G more or less fully, the water more or less quickly reaches its original position at _l_, and acetylene is again produced. Manifestly this arrangement is identical with that of A^2 as regards the periodical immersion of the carbide holder in the liquid; but it is even worse than the former mechanically because there is no rising holder in B^1, and the pressure in the service is never constant. B^2 represents the water store of an unshown generator which works by pressure. It consists of a vessel divided vertically by means of a partition having a submerged hole N. One-half, H, is cloned against the atmosphere, but communicates with the gas space of the generator through L; the other half, K, is open to the air. M is a pipe leading water to the carbide. When gas is being burnt as fast as, or faster than, it is being evolved,