The outlines of physics: an elementary text-book . nauts at a height of 9600 meters = 31,500 ft.,^ andthere are reasons to think that, for at least fifty milesabove the surface of the earth, there is au appreciableatmosphere. 1 At that distance above the sea the barometric column was lessthan 23 cm. long. Life could be sustained, even temporarily, only bysupplementing the scanty supply of air by inhalations of oxygen from acylinder of the compressed gas provided for such an emergency. (Seeaccount of the ascent made by Dr. Berson, Dec. 4,1894 ; Nature, Vol. 51,p. 491.) Fig. 122. PBOPEBTIES OF G
The outlines of physics: an elementary text-book . nauts at a height of 9600 meters = 31,500 ft.,^ andthere are reasons to think that, for at least fifty milesabove the surface of the earth, there is au appreciableatmosphere. 1 At that distance above the sea the barometric column was lessthan 23 cm. long. Life could be sustained, even temporarily, only bysupplementing the scanty supply of air by inhalations of oxygen from acylinder of the compressed gas provided for such an emergency. (Seeaccount of the ascent made by Dr. Berson, Dec. 4,1894 ; Nature, Vol. 51,p. 491.) Fig. 122. PBOPEBTIES OF GASES 143 129. Manometers.—The barometric column may alsobe used to measure changes of pressure artificially pro-duced. If, for example, the cistern of a barometer beplaced upon the plate of an air pump with the tube pro-jecting through the neck of the receiver, as shown inFig. 123, we can reduce the pressure upon the surface ofthe cistern at will. Every stroke of the pump will thenlower the mercury column until the minimum pressure isreached. M. Fig. 123. Fig. 124. Fig. 125. To any such device for measuring artificial pressures, thename manometer is applied. For the measurement of pressures less than one atmos-phere, the form of manometer shown in Fig. 124 is fre-quently used. It is a barometer in which the vacuumabove the mercury column is variable. Where pressures both greater and less than one atmos-phere are to be measured, the open tube manometer (Fig. 144 THE OUTLINES OF PHYSICS 125) is employed. The difference of level {h-Ji^) of themercury in the two arms indicates the amount (+ or —)by which the pressure varies from one atmosphere. For pressures so great that an open tube manometerwould be of inconvenient length, the end of the tube issealed as in Fig. 126, and advantage is takenof the compressibility of air (see Boyles law)to secure a manometer of compact form. Tothis type the name closed tube manometer isgiven. Sometimes, finally, the bending of theflexible top o
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