. The elements of railroad engineering . etically 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
. The elements of railroad engineering . etically 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 held,thus proving that the pressure of air upon it is the same inall Fig. 195. 1069. The W^eight Lifter.—The pressure of theatmosphere is very clearly shown by means of an apparatuslike that illustrated in Fig. 196. Here, a cylinder fittedwith a piston is held in suspension by a chain. At the top ofthe cylinder is a plug A, which can be taken out. Thisplug is removed, the piston pushed up (the force necessarybeing equal to the weight of the piston and rod B) untilit touches the cylinder head. The plug is then screwed in,and the piston will remain at the top until a weight has beenhung on the rod equal to the area of the piston, multipliedby pounds, less the weight of the piston and rod. Ifa force was applied to the rod sufficiently great to force the 426 PNEUMATICS. piston downwards, it would raise any weight less than theabove to the top of the cylinder. Suppose the weight tobe removed, and the piston to be supported, say midwayof the length of the cylinder. Let the plug be removed andair admitted above
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