=73. Automatic Sprinkler System.=—The sprinkler system as now installed for protection against fire in the interior of a building consists essentially of piping connected to a gravity tank and extending over the entire ceiling by means of mains and branches. There is located on the ends of the branches automatic valves or stops, which are collapsed or opened by the melting of a fuse or solder at a temperature more than is likely to exist in the room at any time and still below that which would be created by an incipient fire. =74.= The underlying principles of automatic sprinkler systems as stated by the Underwriters are as follows: 1. Buildings must be open in construction, free from concealed spaces, or places where water thrown from sprinklers cannot penetrate. 2. Sprinklers to be so located that their distribution will cover all parts of the premises. 3. Sprinkler piping to be of sufficient capacity and to have water under pressure in same at all times, except in case of a system where freezing is likely to occur, where an air lock is used. 4. An automatic supply of water of sufficient quantity and pressure available at all times. 5. Systematic, thorough, and intelligent care and inspection of the system. =75. Fireproof Windows.=—It is frequently necessary, and in many cases required by law, and especially recommended by the Underwriters, to provide fireproof window frames and sash in walls exposed to great fire risk, or where it is necessary to admit light into elevator shafts or fire-towers. To meet this demand, several forms of metallic window frames and sashes have been evolved, and these sashes when intended as a fire-retarder are always glazed with wired glass. =76. Wired Glass.=—The wire glass now in common use consists of heavy glass plate with wire mesh embedded in it. This glass is obtainable in polished, ribbed, prism, or mazed form, as shown in Fig. 35 (_a_), (_b_), (_c_), and (_d_), respectively. The plain glass, Fig. 35 (_a_), is used where the light is ample, and where it is desired for the occupants to see through the windows. The ribbed is employed usually in factories, and the ribs are generally run in a horizontal direction, so as to throw the light toward the ceiling and floor, thus diffusing it throughout the building. The prism glass is also employed in order to secure a greater diffusion of the light than is possible with the plain or factory ribbed glass, while the mazed glass finds favor where it is necessary to employ an obscured sash, which will still admit plenty of light and present a good appearance but yet cannot be seen through. The glass used in metallic frames should not be less than ½ inch, or, if polished, ⁵/₁₆-inch, and the embedded wire should not have a mesh larger than 1 inch and should not be less in size than No. 22 Brown & Sharpe wire gauge, which is the standard used in America. [Illustration: FIG. 35] [Illustration: FIG. 36]