Besides the apparatus already touched on, there are pressure-reducing valves on all of the pressure tanks. These may be set by the turning of a handle to any constant pressure required; the dial shows the pressure (Fig. 38).

Both systems have water valves in both gas tubes, to prevent the back pressure of either gas in case of accident. For instance, in the low-pressure system, if the oxygen should accidentally flow back into the acetylene tube, it would get as far as the valve which would let it out into the air instead of into the acetylene tank. This prevents explosions. "The action of hydrolic back-pressure valve is apparent from figure 37. The cocks on the acetylene pipe from the gas holder is connected to the inlet at B, and the acetylene pipe leading to the blowpipe is connected to the outlet C. D is a priming cup through which water can be poured into the chamber until it overflows at the cock F. The cock on the service line at B must be closed while the chamber is being filled with water. When water shows at the cock F, it must be closed and the cock at B opened. The valve is then in working order.

"The pipe G, leading from below the seal at E to priming cup, is made of sufficient length to hold a column of water equal to the pressure in the acetylene holder, which would be equal to not less than 12 inches of water, and in no case should exceed 20 inches.

"In cases where two or more blowpipes are worked from the same acetylene supply pipe, a separate back-pressure valve should be employed for each welding station."

The companies furnish twenty or more feet of hard-rubber tubing, wire-wound.

Goggles for the eyes are advised, both to protect them from the bright light and from flying sparks.