. The elements of railroad engineering . rcury which run outclose the tube and preventair from entering frombelow. These drops ofmercury act like little pis-tons, carrying the air infront of them and forcingit out through the bot-tom of the tube. The airin R expands to fill thetube every time that adrop of mercury falls, thuscreating a partial vacuumin R, which becomes morenearly complete as theprocess goes on. The es-caping mercury falls intothe dish H, from Avhich itcan be poured back into thefunnel from time to the exhaustion from Rgoes on, the mercury risesin the tube c d until, wh
. The elements of railroad engineering . rcury which run outclose the tube and preventair from entering frombelow. These drops ofmercury act like little pis-tons, carrying the air infront of them and forcingit out through the bot-tom of the tube. The airin R expands to fill thetube every time that adrop of mercury falls, thuscreating a partial vacuumin R, which becomes morenearly complete as theprocess goes on. The es-caping mercury falls intothe dish H, from Avhich itcan be poured back into thefunnel from time to the exhaustion from Rgoes on, the mercury risesin the tube c d until, whenthe exhaustion is complete,it forms a continuous col-umn 30 inches high; inother words, it is a barom-eter, whose Torricellian^^- ^^- vacuum is the receiver R. This instrument necessarily requires a great deal of time forits operation, but the results are very complete, a vacuum ofxswo of ^1^ i^ch of mercury being sometimes obtained. Byuse of chemicals in addition to the above, a vacuum of6T0W0 of an inch of mercury has been PNEUMATICS. 425 1067. Note.—A theoretically perfect vacuum is sometimescalled a Torricellian vacuum. 1068. Magdeburg Hemisplieres.—By means ofthe two hemispheres shown in Fig. 195, it can be proventhat the atmosphere presses upon a body equally in all direc-tions. They were invented by Otto Von Guericke, ofMagdeburg, and are called the Magde-burg hemispheres. One of the hem-ispheres is provided with a stop-cock, bywhich it can be screwed on to an airpump. The edges fit accurately and arewell greased, so as to be air-tight. Aslong as the hemispheres contain air, theycan be separated with ease; but when theair in^ the interior is pumped out bymeans of an air pump, they can be sepa-rated only with great difficulty. Theforce required to separate them will beequal to the area of the largest circle ofthe hemisphere (projected area) in squareinches, multiplied by pounds. This force will be the same in whateverposition the hemisphere may be he
Size: 1614px × 1548px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectrailroadengineering
