The accompanying diagrams represent sections of a gas meter, as seen in front and edgewise. The outer case of the instrument, which is a flat cylinder made of sheet iron, is indicated by the letters _c_, _c_. Inside it there revolves another cylinder, made also of thin sheet iron, and divided into four compartments, marked _d_, _d_, _d_, _d_. This interior cylinder readily revolves on an axis, _g_, _g_, shown in the section of the instrument as seen edgewise. The gas enters from the street pipe through the opening, _a_, and it is forced out to the burners through the pipe, _b_, the latter being seen in the narrow section only. In that diagram, also, there is shown a cog-wheel, _h_, fixed on to the axis, and a small outer case, in which that wheel rotates. Water is poured into that external case until the gas-meter is rather more than half filled, the level of the water being shown at _i_. [Illustration] The action of the instrument will be readily understood by examining the two sections. The gas, on entering the tube, _a_, presses against the upper surface of the compartment that happens to be then above it, and tends to turn the inner cylinder round. This pressure forces the gas through the opening, _b_, to the burner; and as the compartment then in communication with that opening is emptied of the gas it contains, in the direction of the arrow, it is gradually forced under the level of the water, and the other compartment, which has in the meantime been filling with gas, continues the supply. Thus, supposing each division of the inner revolving cylinder to hold 108 cubic inches, a complete revolution would indicate that the fourth part of a cubic foot had passed through the pipe, _b_, to the burners. Several cog-wheels, arranged like clock-work mechanism, are connected with the wheel, _g_, and by this means the number of cubic feet of gas consumed is indicated by hands fixed to the wheels, and pointing to the corresponding figures on a series of dials. Some inconvenience and irregularity having been experienced in the use of the wet meter, the correctness of which, it is evident, may be affected by variations in the height of the water level, dry meters have been constructed for measuring gas, by causing it to pass through a small expanding chamber, similar in principle to a pair of bellows. The objection to these instruments is that the leather, or other flexible substance that forms the sides of the expanding chambers, becomes rigid by use, and the valves are liable to get out of order; but in the last improvement of the instrument, by Mr. Croll, these objections are stated to be effectually removed.