The Commissioners appointed by the British Government to construct standard weights and measures, decided that the normal boiling-point, 212 deg., on the thermometer should represent the temperature of steam generated under an atmospheric pressure equal in inches of mercury, at the temperature of freezing water, to 29.922 + (cos. 2 latitude x .0766) + (.00000179 x height in feet above the sea-level). Hence, at London, lat. 51 deg.30' N., we deduce 29.905 as the barometric pressure representing the normal boiling point of water,--the trifling correction due to height being neglected. If then, in the latitude of London, the barometric pressure, at the time of fixing the boiling point, be not 29.905 inches, that point will be higher or lower, according to the difference of the pressure from the normal. Near the sea-level about 0.59 inch of such difference is equivalent to 1 deg. Fahrenheit in the boiling point. Suppose, then, the atmospheric pressure at London to be 30.785 inches, the following calculation gives the corresponding boiling temperature for Fahrenheit's scale:-- Observed pressure 30.785 Normal " 29.905 ------ Difference .880 ======= As 0.59 is to 0.88, so is 1 deg. to 1 deg.5. That is, the water boils at 1 deg.5 above its normal temperature; so that, in this case, the normal temperature to be placed on the scale, viz. 212 deg., must be 1 deg.5 lower than the mark made on the tube at the height at which the mercury stood under the influence of the boiling water. The temperature of the vapour of boiling water may be found, at any time and place, as follows:--Multiply the atmospheric pressure by the factor due to the latitude, given in the annexed Table V., and with the result seek the temperature in Table VI. TABLE V. TABLE VI.