HYPSOM'ETRY. The art of measuring heights on the earth's surface. Sileh measure ments are performed by means of trigonometrical observations and ealeulations; by running a line of precise levels starting from mean sea-level and terminating at the point whose height is to equal to one mile, then, the dimension on is 0.007 foot, due to curvature minus one-seventh of 0.607 foot due to refraction, which makes it 0.5714 foot. In careful geodetic work the calculations for re fractions and curvature are made with much more precision than is done above and the instrument observations are made with exceeding care, but the general method is the same. At hest such measurements are only approximate, since the de termination of the refractions, which depends upon the atmospheric conditions. can be approxi mate only. To measure the height SR by leveling, the observer starts from mean sea-level and runs a line of precise levels inland, which terminates at the point T. and establishes accurately its height above mean sea-level. The some result may he ne•omplished by starting the line of levels to x from a point on a line of levels previously run from mean sea-level. The method of running precise levels differs from ordinary spirit level ing (see LEVELING) only in the greater accuracy of the methods and instruments used.
For measuring heights by barometric Lions the form of barometer known as the aneroid barometer is usually used. :Mercury barometers may of course be used, but usually they are not, owing to the greater difficulty in transporting them and of keeping them in proper working order. The adaptability of the barometer as an instrument for measuring heights depends upon the facts that the mercury column falls as the atmospheric pressure decreases, and that the atmospheric pressure decreases gradually as we ascend above the sea-level. As a rough average
it may he assumed that the barometer falls 1-10 inches for about 106 feet rise. To measure the height sx, therefore, barometer readings are taken simultaneously at s and at x, and from them the difference in height of s and x may be found by the formula, — \ d = 6000 (log. H—log. h) + T 9 t 0 0 60 ) in which d equals the difference in height ; II and h are the two readings. and T and t are the tem perature of the two stations in degrees Fahren heit. In the most accurate barometric work va rious refinements of observation and calculation are introduced, but the general principle of the operation is the same as has been described. Barometrical measurements of heights are only approximate, since the moisture and dryness of the air, the wind, and various other atmospheric phenomena cause variations in the readings re corded. and no formula can possibly be devised that will embrace all these sources of error. Simultaneous observations, with barometers ad justed to the same standard, give the most re liable results. For a full discussion of the methods of measuring heights by trigonometrical leveling, precise leveling, and barometric observa tions. the reader many consult Johnson, Theory and Practice of Surveying (New York, 1900). See SURVEYING.