CHAPTER XXV.
PRESSURE.
The variations of atmospheric pressure, although insensible to non-instrumental observation, are so intimately connected with atmospheric processes that they deserve careful attention. Their observation leads to several problems.
A. The Decrease of Pressure with Height, as between Valley and Hill, or between the Base and Top of a Building.—Make these observations with the mercurial barometer, if possible. Note the air temperatures at the two levels at which the barometer readings are made. Determine the heights of hill or building by means of the following rule: Multiply by 9 the difference in barometrical readings at the two stations, given in hundredths of an inch, and the result will be approximately the difference in height between the stations in feet. A more accurate result may be reached by means of the following rule: The difference of level in feet is equal to the difference of the pressures in inches divided by their sum, and multiplied by the number 55,761, when the mean of the air temperatures of the two places is 60°. If the mean temperature is above 60°, the multiplier must be increased by 117 for every degree by which the mean exceeds 60°; if less than 60°, the multiplier must be decreased in the same way. For example, if the lower
station has a pressure of 30.00 inches and a temperature of 62°, and the upper station has 29.00 inches and 58° respectively, the difference of level between the two will be
(30.00 - 29.00) / (30.00 + 29.00) × 55,761 = 945 feet.
If the lower values are 30.15 inches and 65°, while the upper values are 28.67 inches and 59°, then the formula becomes
(30.15 - 28.67) / (30.15 + 28.67) × [55,761 + (2 × 117)] = 1409 feet.
The determination of heights by means of the barometer depends upon the fact that the rate of decrease of pressure upwards is known. As the weight of a column of air of a given height varies with the temperature of the air, it is necessary, in accurate work of this sort, to know the air temperatures at both the lower and upper stations at the time of observation. From these temperatures the mean temperature of the air column between the two stations may be determined. Tables have been published which facilitate the reductions in this work. The heights of mountains are usually determined, in the first instance, by means of barometric observations, carried out by scientific expeditions or by travelers that have been able to reach their summits. More accurate measurements are later made, when possible, by means of trigonometrical methods.