The _water-gauge_, _P_, fig. 71, which is shown in section in fig. 73, consists of an upright[34] glass tube, _a a_, which is from one-half to three-quarters of an inch in diameter, and from 12 to 15 inches long. The glass is about one-eighth of an inch thick. At its ends it communicates with the steam and water of the boiler through brass elbows, _b_, _c_. The openings in these elbows, which communicate with the boiler, are closed by the valves or plugs, _d_, _e_, which are worked by screws and handles, _f_, _g_. The glass tube, when it is attached to the elbows, is made steam-tight by rubber rings, which are pressed tight around the tube by _packing-nuts_, _h_, _i_. The elbows are provided with the valves, _d_, _e_, so that in case the glass tube breaks the steam and water can be shut off, so as not to escape through the elbows. The lower elbow is provided with a blow-off cock, _k_, through which any sediment or dirt which collects in the glass tube or elbows can be blown out. When the valves in the upper and lower elbows are opened the steam flows into the glass tube through the upper one, and water through the lower one, and the water assumes a position in the glass tube on a level with the surface of that in the inside of the boiler; that is, the position of the water in the boiler becomes visible in the glass tube. On account of the constant variations of the water in the boiler, the column of water in the glass never remains stationary, but plays up and down as long as the boiler is working. But if the communication between the glass tube and the boiler is closed, then the water in the tube becomes stationary and the water gauge is useless. In order that there may be no obstruction of the glass tube by mud or dirt from the water, it must be _blown out_ often. To do this the lower valve, _e_, is closed, and the blow-off cock, _k_, and the steam valve, _d_, are opened. The steam pressure in the tube on top of the column of water will force it out of the blow-off cock, and the mud and dirt will be carried with it. [34] Sometimes these tubes are, for convenience, inclined, as shown in fig. 71. If from any cause the glass tube is broken, first of all the water-valve _e_ should be closed and then the steam-valve _d_, so as to prevent the hot water and steam which will escape from the broken glass from scalding those who are working the engine. By unscrewing the nuts _h_ and _i_ the old glass can easily be removed and a new one substituted in its place.[35] Care should be taken in putting in new glasses not to screw the packing nuts down any more than just sufficiently to make the rubber rings steam-tight around the glass tubes. If they are screwed too tight they are apt to produce a strain on the tube, so that the slightest expansion by heat or contraction from cold will break it. [35] Extra glasses should always be carried with an engine so as to be substituted in case of accident to the one in use. [Illustration: Fig. 74.