HYPSIPYLE. 410 HYPSOMETRY. Adrastus nnd llie otlipr heroes wore on their way to the siege of Tlicbes, they met Ilypsipyle in the Xemean forest and lie^iged her to show them a spring, and iluring lier absence the boy Avns killed by a serpent. Fur this neglect she was imprisoned, but was rescued by her sons. The funeral games instituted by the heroes for the dead Oidudtes were the origin of the Nemean CJamcs. HYPSISTA'RIANS (from Gk. "C^urTipuii. Ilypsisluriui, llypsislarians, from f^irrror , hyi'- nistos, most liigh. from ti^i, /ii/;<si, on high). A small sect which existed in the fourth century in Asia Minor. It seems to have been mono- theistic and .Jewish in .some of its features, and in others Christian, debased by Sabean elements. Thus it taught the unity of the Ueity. and His imiversal dominion; that sacrifices and other phe- nomena of external worship were to be given up; at the same time it enjoined Sabbath observance and the dietary laws of the Jews. HYPSOM'ETER (from Gk. t/^(, hypsi, on high + liiTpov, iHClron, measure), or Tiieujmo- Babometku. 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 abovo the sea-level. With the barometer (q.v.) it is possible to measure heights with considerable exactness; consequently, by knowing the atmos- pheric pressure corresponding to the temperature at which water boils, we are able to find the altitude. The apparatus to determine the boil- ing-point was first used by WoUaston in 1810, and consisted of a metal vessel containing 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 mercurial 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 calculated for a mean temperature: be measured, and by barometric observations nnd 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 methods of measurement a supposititious case will be assumed and 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 trigo- nometrical observation, and whose distance mi from the point of observation is known. The instrument is set up at y and by it the vertical angle inix is measured. This gives the obser- ver a triangle mix, of which the side mi, the angle imx, and the angle mx (!)0°) are known, and from these data he can readily calculate the length of the side )>. To this dimension in order to get the height of the mountain he must add Ihe dimension io, which is the height of the in- strument above the ground, and the dimension o», which is due to the curvature of the earth. The dimension io is easy to determine, but on luvs to be calculated. Xow the curvature of the earth is 0.0G7 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.G07 foot. Owing to the re- fraction of the atmosphere, however, the ])oint 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 cur'a- ture of the earth, and, like the latter, varies with the square of the distance. For a distance mi BollinB-poInt (CentiKrade). Height of barometer(mm.). Elevation (in meters) 100° 99° 98° 97° 96° 95° 94° 93° 92° 9I» 760 732.2 707.1 681.9 657.1 633.7 610.6 588.3 666.7 546.8
305 610 920 12i5 1540 1860 2166 2486 2800 90° 525 3120 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 mm., at an elevation of 1000 meters 2.5 ram., at 2000 meters 2.2 mm., and at .3000 meters 2 mm. An approximate rule to determine the elevation of a place by this method, ex- pressed in feet and Fahrenlieit degrees, is to mul- tiply the dilference between the observed boiling- point and 212° by 5'tO. if the height is less than half a mile, and by .ifiO if it is in exc^ess of that amount, these factors representing the average amount of altitude corresponding to 1° at 70' Fahrenlieit. HYPSOM'ETRY. The art of measuring heights on the earth's surface. Such measure- ments are performed by means of trigonometrical observations and calculations; by running a line of precise levels starting from mean sea-level nnd terminating at the point whose height is to equal to one mile, then, the dimension os is 0.667 foot, due to curvature minus one-seventh of 0.007 foot due to rcfr.a<tion, which makes it 0..'j7I4 foot. In careful geodetic work the ealculatiims for re- fractions and curvature are made with much more precision than is done above and the instrument observations are made with exceeding eare, but the general method is the same. At Iwst 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 xs by leveling, the observer starts from mean sea-level and runs a line of precise levels inland, which terminates at the point x, and establishes accurately its height above mean sea-level. The same result may be accomplished by starting the line of levels to x from a point on a line of levels prcvioisly nm from mean sea-level. The method of running precise levels dilTers from ordinary spirit level-
