Every locomotive is provided with four or more gauge-cocks, which are usually placed at the back end of the boiler, where they can easily be seen and reached. These cocks, _s_, _s_, _s_, _s_, are shown in fig. 71, which represents the back end of a locomotive, and to which frequent reference will be made. They are also shown on a larger scale in fig. 72, which represents the end plate of the boiler in section. They communicate with the inside of the boiler and are so placed that one is three or four inches above the other. The two upper cocks are placed above the point where the surface of the water should be when the engine is working, and the two lower ones below it, so that the upper ones communicate with the steam space and the lower ones with the water. When these cocks are opened, if the water is at its proper height, steam is discharged from the two upper ones, and water from the two lower ones. When a gauge-cock which communicates with the steam space is first opened, it is usually filled with condensed water, so that it should usually be kept open for a little while until this water is discharged. If the upper cocks are opened and continue to discharge _water_, they indicate that there is _too much_ water in the boiler; on the other hand, if steam is discharged when the lower cocks are opened, then there is too little water in the boiler, and the heating surface is in danger of being exposed to the fire without being covered with water, and consequently overheated, or as it is called “burned,” and so injured as to become too weak to bear the strain to which it is subjected by the pressure of the steam. There is then great danger that the crown sheet may be crushed down by the pressure of the steam above it, or that the boiler may be exploded. Even if no accident occur, the boiler is in great danger of permanent injury from overheating when the water is allowed to get too low. Below the gauge-cocks _s_, _s_, _s_, _s_, fig. 71, an inclined cylinder, _R_, called a _drip-pipe_, is placed with openings to receive the water and steam which are discharged from the cocks. This water is conducted away by the pipe _p_.