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. _B._ =The Diurnal and Cyclonic Variation of Pressure in Different Seasons.=——This problem is satisfactorily solved only by a study of the curves traced by the barograph, or by plotting, as a curve, hourly or half-hourly readings of the mercurial barometer. The _diurnal variation of the barometer_ is the name given to a slight double oscillation of pressure, with two maxima and two minima occurring during the 24 hours. This oscillation is in some way, not yet understood, connected with the diurnal variation in temperature. It is most marked in the tropics and diminishes towards the poles. Fig. 15 illustrates, in the May curve, the diurnal variation of the barometer at Cambridge, Mass., during a spell of fair spring weather, May 18-22, 1887. The maxima are marked by + and the minima by 0. The _cyclonic variation of pressure_ is the name given to those irregular changes in pressure which are caused by the passage of cyclones and anticyclones. The second curve in Fig. 15 shows the cyclonic variations in pressure recorded by the barograph at Cambridge, Mass., during a spell of stormy weather, Feb. 23-28, 1887. These curves serve as good illustrations of these two kinds of pressure variations. Study your barograph tracings, or your barometer readings, as illustrating diurnal or cyclonic variations of pressure. Note the character and the amount of the diurnal and cyclonic variations, and their dependence on seasons. Over the greater part of the Torrid Zone the diurnal variation of the barometer is remarkably distinct and regular. Humboldt first called attention to the fact that in those latitudes the time of day may be told within about 15 minutes if the height of the barometer is known.