_The try-cocks_ _T_ and _T_ (Fig. 18) are also intended to indicate the approximate height of the water in the boiler and should the water glass be broken may be used in its place. The openings of the try-cocks point toward the floor. When a cock is opened, should steam alone escape, it will be absorbed by the air, but if water is escaping, although much of it will be vaporized and look like steam, some of the water will be carried to the floor and produce a wet spot. When the cock is opened wide the escaping water from the lower cock should always wet the floor. _The drip-cock_ _P_ (Fig. 18) at the bottom of the gage-glass is for draining the water column and for blowing out any deposit that may collect in the opening of the column. This cock should be opened occasionally to assure the correctness of the gage-glass. [Illustration: FIG. 20.--Typical Bourdon pressure gage with the face removed.] =The Steam Gage.=--Steam pressure is measured in pounds to the square inch above the pressure of the atmosphere. The gages used for indicating the pressure of the steam are made in several forms but the type most commonly used is that shown in Fig. 20. It is known as the Bourdon type of gage and takes its name from the bent tube _A_, which furnishes its active principle. The Bourdon barometer invented in 1849 employed this form of sensitive tube. In the drawing the face of the gage has been removed to show the working parts. The sensitive part is the flat elastic tube _A_, which is bent in the form of a circle. When the pressure of the steam enters at _S_ the air in the tube is compressed and the tube tends to straighten. The movement of the tube caused by the steam pressure is communicated to the pointer by a link connection and gear as shown in the drawing. The amount of straightening of the tube will be in proportion to the steam pressure and is indicated by the numbers marked on the face of the gage. When the pressure is released, the tube returns to its original position and the spiral spring _C_ turns the hand back to its first position. [Illustration: FIG. 21.--Cross-section of a pop valve.]