In order to show the height of the water in the boiler, two methods are employed; the first consisting of a number of gauge cocks fixed at 2 or 3 inches apart, one over the other, by turning which in succession, the engineer ascertains whether they are open to water or steam: the other is by an instrument called the water gauge, which consists of a strong glass tube a, about three quarters of an inch diameter outside, fitted into a brass socket b b at top and bottom, the joints being made steam tight by hemp packing put round the glass, and compressed against it by the glands c c, which are screwed in round the glass. From each of the socket pieces b a tube d proceeds with a cock in it, and a screw on the end, for fixing it into the fire-box, and the piececcontaininganother cock is screwed into the lower piece, and the plug / into the upper piece, affording the means of putting the glass tube down into its place. When the two cocks in d d are opened, the water of the boiler rises in the glass tube to the same height that it is in the boiler, the upper part of the glass being filled with steam, and so remains, shewing always the level of the water in the boiler; the cocks are for the purpose of stopping the communication when required, from the gauge being out of order or otherwise.
The cock in the piece e is for the purpose of clearing out the gauge, by allowing a stream of water to run through it, and it is often necessary to open it when examining the gauge, in order to get rid of the bubbles of steam formed by the rapid ebullition of the water, which sometimes render it difficult to ascertain the precise height of the water. The difficulty is also increased by the motion of the engine, producing oscillation in the water, but the disturbing effect is much diminished by choking the tube or making the communication with the boiler through the tube d very small, so as to impede the motion of the water in the tubes. A small plug g is screwed in opposite each tube d, to afford the means of clearing out the tubes d, by passing a wire through them when the plugs g are taken out.