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Page:Popular Science Monthly Volume 57.djvu/191

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flow of gas along the line will vary from hour to hour, consequently the pressure at his house may be expected to vary from hour to hour.

The unit of measurement of gas pressure is that pressure which will cause a difference of water level of one tenth of an inch in the two legs of a V-shaped tube when one end is connected with the gas main and the other end is left open to the outer air. Ten tenths, or one inch, is the standard, or normal pressure.

Any appliance—even a gas-burner—operates to best advantage

PSM V57 D191 Pressure recorder indicating normal daytime gas use.png
Exhibit 1.

under certain well-defined conditions. Depart from these conditions, and the efficiency of the device is impaired to an extent depending largely upon the nature of the appliance under consideration. For example, burn an incandescent lamp at fifty per cent, above normal voltage and it breaks down; burn a gas jet at two hundred per cent, above normal pressure, and it still operates—how satisfactorily 'deponent sayeth not.' Now, the gas-burner is supposed to operate to best advantage at ten tenths of an inch. At this pressure the flame is neither so wavering as to be affected by every chance draught, nor so rigid as to