The gas made on a gasworks is always measured by the station meter, and in modern establishments corrections are made for temperature and pressure, in order that the gas may be measured under standard conditions, since, as the height of the barometer, and more especially the temperature of the atmosphere, varies at different seasons of the year, the measurement of the gas is affected in accordance with the atmospheric conditions prevailing; hence, in practice, the volume of gas passing through the station meter is always reduced to the standard conditions of 60 F. and a barometrical pressure of 30 in. of mercury. The calculations are based upon the following physical laws. By the law of Boyle or Mariotte, the volume of a given mass of any gas, assuming that the temperature is constant, varies inversely as the pressure to which it is subjected; or, in simple language, doubling the pressure reduces the volume to one half, while, conversely, reducing the pressure one-half doubles the volume, and so on in a similar ratio.

Now, supposing a station meter registered 10,000 cub. ft. of gas under a, barometrical pressure of 30.5in., and we wished to reduce the volume to the standard pressure of 30 in., since the pressure under which the gas is measured is greater than the standard pressure (30 in.), it is plain that under the last-mentioned pressure the volume would be greater; consequently, we say:

As 30: 30.5:: 10000: 10160 cub. ft. Or, supposing that we measure the same volume of gas under a pressure of 29.5 in.,and we wished to know the volume at the standard pressure; in this case, the gas is measured under a lesser pressure than the standard, consequently, when reduced to the latter pressure, the volume would be reduced; so in this case we say,

As 30: 29.5:: 10000: 9833 cub. ft. It will be noticed that in each case the standard pressure (30 in.) occupies the first term in the statement. With regard to temperature, as is well known, gases expand with heat and contract with cold, and the amount of this is expressed as follows. The volume of a gas expands or contracts by 1/492 part of its volume at 320 F. for every increase or decrease of 1° F. Now supposing we measure I0,000 cub. ft. of gas at a temperature of 80 F., and we wish to correct it to the standard temperature of 60 P. (the pressure remaining constant), 192 volumes at 320 F. become 492 + (60 - 32) =520 volumes at 600 F., and 492 + (80 32) = 540 volumes at 80 P. The volume, therefore, of any gas at 80° F. would bear the same ratio to the volume which it would occupy at 600.F., as 540 does to 520; consequently:

As 540: 520:: 10000: 962!) cub,. ft.

If the gas, instead of being measured at 80 F.,had been measured at 400 F.,then,as before, 492volumes at320 F. would become 520 volumes at 600 F., and 429 volumes at 32' 1'. would become 492 + (40 - 32) 500 volumes at 400 F. Then the ratio of the volume at 600 F. would be obtained as follows:

As 500 : 520:: 10000: 10400 cub. ft.

It will be noticed that 520 always occupies the second term in the proportion. In practice, the volume of a gas is always corrected for temperature and pressure at one operation by combining the two corrections and making a compound proportion sum of it, and as two of the terms always occupy the same position, by cancelling we obtain this expression -

(17.333xpxv)/460+t = corrected volume, p being the pressure under which the gas is measured, V the volume, and t the temperature under which the gas is measured. In gasworks, however, these corrections are usually performed by means of a series of tables drawn up by the Metropolitan Gas Referees, based on the principles already explained, but also taking into account the tension of aqueous vapour, the formula from which their numbers are obtained being - 17.61 (b - a) x V 460 + t a representing the tension of aqueous vapour to be deducted from the height of the barometer according to the temperature under which the gas is measured, while 17.64 only differs from the 17.333 previously given by deducting from 30 the tension of aqueous vapour at 600 F. By the aid of these numbers all that is required is to observe the temperature of the thermometer at the inlet of station meter, and the height of the barometer, then find the number corresponding to them, and multiply the volume of gas by the number, when the corrected volume at 60° F. and 30 in. will be obtained.