Tri-Cocks - Tri-cocks are small, simple angle valves used to release small quantities of water or steam, for the purpose of discerning if the water level in the boiler is above or below that of the tri-cock. Most engine boilers were equipped with two or three tri-cocks, with the middle (3 cocks) or upper (2 cocks) being located where the manufacturer thought was the optimum water level, and the lower cock at a minimum water level. In use, the operator merely cracks the valve open, and looks at what comes out. A jet of hot water indicates that the water level is above the cock, a plume of steam indicates that the water level is below the cock. A mixture indicates that the water level is approximately at the cock's level. The tri-cocks can also all be opened slightly and left open, with the drips and/or steam wisps giving a constant indication, although this can lead to problems with the tri-cocks liming up, and the constant flow can erode the valve seats.
    Tri-cocks should be tested once a day to check against the sight glass, and to ensure proper working. Since the sight-glass gives an easier, more accurate reading, the tri-cocks tend to be viewed as a safety backup method of checking the water level.