HYPSOMETRY, that department of geodesy which treats of the measure ments of the altitudes or relative heights of various points on the earth's surface. In all. cases in which great accuracy is essential, trigonometrical methods must be employed, but in other cases suffi ciently accurate results may be obtained by levelling, by the use of the barometer, or by the boiling point of water as given by the thermometer. The trigonomet rical method is often the only one avail able, as the height to be measured may be quite inaccessible. The barometric method is based on the fact that as the mercurial column is supported by the atmospheric pressure, it must fall when conveyed from a lower to a higher level, as in the latter case the pressure is diminished. Were the atmosphere uni form in density throughout, nothing could be simpler than the measurement of heights by the barometer, but gases being very compressible, the lower strata of the atmosphere are denser than the upper strata, being exposed to greater pressure. Moreover, increase of tem
perature affects the density of the mer cury in the barometer, and also that of the air, and further complicates the problem. Hence for the greatest accu racy in determining the difference of levels two mercurial barometers and four thermometers are required. Two of the thermometers are used for determin ing the temperature of the air at the stations, and two are attached to the barometers for determining the tempera ture of the mercury. The observations are made simultaneously. The aneroid barometer is in some respects more suit able than the mercurial, being much more portable, and requiring two ther mometers only. After the necessary ob servations are made the required height may be calculated by the use of certain logarithmic formul.