PNEUMATICS. 



the fluid is confined within it, and has 

 no opening through which it can escape. 

 Let a hole be cut any where in this ves- 

 sel of any proposed magnitude, as a 

 square inch, and let the piece cut out 

 be imagined to be replaced by a solid 

 piston fitting the hole, so that the fluid 

 cannot escape between it and the sides 

 of the hole. We shall suppose the fluid 

 inelastic. Let a pressure equal to one 

 pound weight urge the piston inwards. 

 Such is the peculiar nature of fluidity, 

 that a pressure of one pound will be 

 exerted on every square inch of the 

 inner surface of the vessel, so that by 

 an actual pressure amounting to one 

 pound, an effective pressure of as many 

 pounds as there are square inches on 

 the inner surface of the vessel will be 

 thus produced. This perfect power of 

 transmitting pressure is the specific 

 attribute of fluids whether elastic or in- 

 elastic, and it is the mechanical property 

 which forms the basis of all mathemati- 

 cal treatises on the theory of fluids. 



(14.) We shall now enter more mi- 

 nutely into the consideration of the 

 weight of the atmosphere. 



Let A B (Jig. 5.) be a glass tube up- 



Glass cistern filled to C D 

 with quicksilver. 



v;ards of thirty-two inches in length, 

 open at one extremity A, and closed at 

 the other, B. The tube having been 



carefully cleaned on the inside, let a 

 quantity of mercury (quicksilver,) well 

 cleansed and purged of air by boiling, 

 be provided. Turning the closed end B 

 of the tube down, let it be filled with 

 the mercury through the open end A. 

 Let a small cistern C D be also provided, 

 and filled with mercury to the height 

 C D. Placing the finger firmly on the 

 end A, so as to prevent the mercury 

 from escaping out of the tube, let it be 

 inverted, and the open end A plunged 

 in the vessel of mercury. When the 

 mouth A of the tube is below the sur- 

 face C D of the quicksilver in the cistern, 

 let the finger be removed from the aper- 

 ture A, the mercury in the tube will 

 then be observed to fall to the height E, 

 about twenty-nine or thirty inches above 

 the surface C I), and there, after a few 

 vibrations, it will rest. 



It must no doubt excite inquiry, why 

 the column F E of mercury remains 

 suspended in the tube, and why, as 

 might naturally be expected, the surface 

 E does not fall to the level D C of the 

 mercury in the cistern ? A little consi- 

 deration will, however, solve this diffi- 

 culty. It will be remembered, that the 

 tube being closed at B, the space B E is 

 a perfect void, in which there is neither 

 air nor any other fluid. The column of 

 mercury E F therefore presses with no- 

 thing but its own weight on the level 

 C F D of the mercury in the cistern ; for 

 in this pressure the weight of the atmos- 

 phere has no part, since it is excluded 

 from above the surface of the mercury 

 E. The pressure thus exerted at F, by 

 the weight of the column E F, is, by 

 the property of the liquid mercury de- 

 scribed in (13), transmitted to the exte- 

 rior surface C F of the mercury in the 

 cistern, and gives that surface a tendency 

 to rise with an equivalent force. That 

 this surface would rise is certain, w r ere 

 it not resisted by a force accurately equal 

 and opposite to that we have just men- 

 tioned. This force is the weight of the 

 atmosphere itself resting on the surface 

 C F. Thus, then, it appears that the 

 atmosphere must necessarily press on 

 the surface CF with a force exactly 

 equal to that with which the weight of 

 the column of mercury F E presses on 

 the level F. 



If we were to suppose the base of the 

 column FE to be equal to a square inch, 

 it would therefore follow that the at- 

 mosphere presses on every square inch 

 of the surface of the mercury in the cis- 

 tern CD, with a force equal to the 



at 



