HYPSOM'ETER (from Gk. qt, )typsi, on high + tarpop, outrun, Pleasure), or THERMO 13.kuomETEn. An instrument to determine the at mospheric pressure and altitude at a given point by ascertaining accurately with a thermometer the temperature at which water boils. The boil ing-point of water diminishes with a decrease in the pressure of the atmosphere, and this al ways comes with an increase in elevation above the sea-level. With the barometer (q.v.) it is possible to measure heights with considerable exactness; consequently, by knowing the atnws pherie pressure corresponding to the temperature at which water boils, we are able to lind the altitude. The apparatus to determine the boil was first used by Wollaston in 1816, and consisted of a metal vessel eontaining a delicate and carefully tested thermometer with graduations extending from 80° to 100° C., which could be read to .01 of a degree. There is a spirit lamp to boil the water, and the instrument is far more portable than a im•reurial barometer. The relation between the boiling-point of water, the height of the barometer. and the altitude is shown in the following table, which is for a mean temperature: • he measured, and by barometric observations and calculations. The second of these methods of measurement gives exact heights, but the other two methods give approximate heights only. To illustrate the application of the three na.thods of measurement a supposititious ease will be assumed each method applied to its solution. Let x represent the top of a mountain whose height above the base s it is desired to measure by nometrical observation, and \•hose distance tnf from the point of observation is known. The instrument is set up at and by it the vertical angle jinx is measured. This gives the obser
ver a triangle inix, of which the side mi. the angle jinx, and the angle our (00°) are known. and from these data he can readily calculate the length of the side ix. To this dimension in order to get the height of the mountain he must add the dimension io, which is the height of the in strument above the ground, and the dimension os, is due to the curvature of the earth. The dimension io is easy to determine, but os hax to Ire calculated. Now the curvature of the earth is 0.007 foot, or nearly 8 inches in one mile, and increases as the square of the distances, being thus :32 inches in two miles and 72 inches in three miles. If then the distance mi is one mile, the dimension os is 0.607 foot. Owing to the re fraction of the atmosphere, however, the point x always appears to the observer to be higher than it really is, and at an average this deceptive elevation amounts to one-seventh of the curva ture of the earth. and, like the latter, varies with the squara of the distance. For a distance uti From this table it will be seen that a variation of .1° in the boiling-point at sea-level corre sponds to a decrease in the height of the barom eter of 2.7 min.. it an elevation of 1000 meters 2.5 min.. at 2000 meters 2.2 min., and at 3000 meters 2 non. .\11 approximate rule to determine the elevation of a place by this method. ex pressed in feet and Fahrenheit degrees, is to mul tiply the difference between the observed boiling point and 21'2' by 550. if the height is le-s than half n mile, :111(1 by 5110 if it is in excess of that amount, those factors representing the average amount of altitude corresponding to I° at 70° Fahrenheit,