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eight with water. The vacant upper half of one limb is put in communication with the gas-supply of which the pressure is to be determined, while the other limb remains open to the atmosphere. The difference then observed, when the U-tube is held vertical, between the levels of the water in the two limbs of the tube indicates the difference between the pressure of the gas-supply and the atmospheric pressure. It is this difference that is meant when the pressure of a gas in a pipe or piece of apparatus is spoken of, and it must of necessity in the case of a gas-supply have a positive value. That is to say, the "pressure" of gas in a service-pipe expresses really by how much the pressure in the pipe exceeds the atmospheric pressure. (Pressures less than the atmospheric pressure will not occur in connexion with an acetylene installation, unless the gasholder is intentionally manipulated to that end.) Gas pressures are expressed in terms of inches head or pressure of water, fractions of an inch being given in decimals or "tenths" of an inch. The expression "tenths" is often used alone, thus a pressure of "six-tenths" means a pressure equivalent to 0.6 inch head of water.
The pressure gauge is for convenience provided with an attached scale on which the pressures may be directly read, and with a connexion by which the one limb is attached to the service-pipe or cock where the pressure is to be observed. A portable gauge of this description is very useful, as it can be attached by means of a short piece of flexible tubing to any tap or burner. Several authorities, including the British Acetylene Association, have recommended that pressure gauges should not be directly attached to generators, because of the danger that the glass might be fractured by a blow or by a sudden access of heat. Such breakage would be followed by an escape of gas, and might lead to an accident. Fixed pressure gauges, however, connected with every item of a plant are extremely useful, and should be employed in al