Heights

Heights, Determination op. This may be effectea by calculation from data supplied by ordinary surveying instruments, the necessary formula? being supplied by trigonometry. Also it may be obtained by careful measurement of the barometric pressure at the unknown height, if at the same time the pressure at another known level in the vicinity be observed. This barometric pressure is usually in accurate work taken with a mercurial barometer. Such as are employed for the determination of the heights of mountains are specially constructed to admit of transport without danger of fracture. Aneroid barometers (q.v.) may also be used; they are far more portable and less liable to damage, but they are less accurate, and to get satisfactory readings it is necessary to have closely studied the behaviour of the special instrument employed under various conditions of pressure and temperature. Scales of height are often attached, but theory requires that a separate scale should be used for every value of barometric pressure at sea-level. Hence the results of using such a scale are only approximately correct, perhaps strictly so in the one case where zero-level pressure is 31 inches of mercury. A third way of determining the pressure, and of therefore estimating the height, is by observation of the boiling point of water. The temperature at which this takes place is 212° F. at a pressure of 30 inches of mercury; but as this temperature becomes lower when the pressure is lowered, and as the relation between the pressure and the boiling-point is accurately known the above object may be readily obtained. A portable piece of apparatus for supporting a sensitive thermometer in the vapour proceeding from boiling water is constructed for height-measurements and is known as the hypsometer.