178 



"WELLS'S NATURAL PHILOSOPHY. 



Fig. 169. 



brass, Fig. 168, which fit together air-tight. By exhausting the air in their 

 interior, by means of the air-pump, and a stop-coek arrangement affixed to 

 one of the hemispheres, it will be found that they can not be pulled apart 

 without the exertion of a very great force, since they will be pressed to- 

 gether with a force of 15 pounds for every square inch of their surface. 

 In the exhibition above referred to, given of these hemispheres by Guericke, 

 the surfaces of a pair constructed by him were so large, that thirty horses, 

 fifteen upon a side, were unable to pull them apart. By admitting the air 

 again to their interior, the Magdeburg hemispheres fall apart by their owa 

 weight. 



Another interesting example of atmospheric pressure is, 

 to fill a wine-glass, or tumbler with water to the brim, 

 and, having placed a card over the mouth, to invert it 

 cautiously. If the card be kept in a horizontal position, 

 the water will be supported in the glass by the pressure 

 of the air agauast the urface of the card. (See Fig. 169.) 



384. In a like manner, if we take a 

 principle and jar, and having filled it with water, in- 

 construction of ^^j.^ j^. jjj ^ reservoir or trough, as is rep- 

 resented in Fig. 170, it will continue to bo 



completely filled with water, the It 

 quid being sustained in it by the pres- 



the gasometer. 



Fig. 170. 



sure of the atmosphere upon the water 

 in the vessel. Such an arrangement 

 enables the chemist to collect and pre- 

 serve the various gases without admix- 

 ture with air; for if a pipe or tube 

 through which a gas is passing be 

 depressed beneath the mouth of the 

 jar, so that the bubbles may rise into 

 it, they will displace the water, and be 

 collected in the upper part of the jar, 

 free of all admixture. 



The gasometers, or large cylindrical 

 vessels in which gas is collected in 

 gas-works for general distribution, are 

 constructed on this principle. They 

 consist, as is shown in Fig. 171, of a 

 liarge cylindrical reservoir suspended with its mouth downward, and plunged 

 in a cistern of water of somewhat greater diameter. A pipe which leads 

 from the gas-works is carried through the water, and turned upward, so as to 

 enter the mouth of the gasometer. The gas, flowing through the pipe, rises 

 into the gasometer, filling the upper part of it, and pressing down the water. 

 Another pipe, descending from the gasometer through the water, is continued 

 to the service pipes, which supply the gas. The gasometer is balanced by 

 counter weights supported by chains, which pass over pulleys, and just such 



