oz., or less than half a pound per square inch--is amply sufficient for every kind of installation, the less meritorious generators with displacement holders only excepted. This pressure, it should be noted, is the net or effective pressure, the pressure with which the gas raises the liquid in a water-gauge glass out of the level while the opposite end of the water column is exposed to the atmosphere. The absolute pressure in a vessel containing gas at an effective pressure of 12 inches of water is 7 oz. plus the normal, insensible pressure of the atmosphere itself--say 15-1/4 lb. per square inch. The liquid in a barometer which measures the pressure of the atmosphere stands at a height of 30 inches only, because that liquid is mercury, 13.6 times as heavy as water. Were it filled with water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet, approximately. Gas pressures are always measured in inches of water column, because expressed either as pounds per square inch or as inches of mercury, the figures would be so small as to give decimals of unwieldy length. It would of course be perfectly safe so to arrange an acetylene plant that the pressure in the generating chamber should reach the 100 inches of water first laid down by the Home Office authorities as the maximum allowable. There is, however, no appreciable advantage to be gained by so doing, or by exceeding that pressure which feeds the burners best. Any higher original pressure involves the use of a governor at the exit of the plant, and a governor is a costly and somewhat troublesome piece of apparatus that can be dispensed with in most single installations by a proper employment of a well-balanced rising holder. CHAPTER III THE GENERAL PRINCIPLES OF ACETYLENE GENERATION--ACETYLENE GENERATING APPARATUS