306 LIGHTING BY GAS. 



pressing upon the hand when the man holds it out hori- 

 zontally before him is not less than twenty times fifteen 

 pounds that is, three hundred pounds ! Now, if this down- 

 ward pressure upon the upper surface were not balanced 

 and counteracted by an equal upward pressure upon the 

 under surface, and also from certain resisting pressures ex- 

 erted by the fluids within the hand, no man could hold his 

 hand out horizontally in that manner for a moment. 



We see what the prodigious force of this pressure is, 

 when not counterbalanced, by the action of certain steam- 

 engines which are worked on the principle of producing a 

 vacuum upon each side of the piston in the cylinder alter- 

 nately, by which means the counterbalancing pressure is 

 taken off, and the pressure of the air on the other side is al- 

 lowed to act without any thing to oppose it. By this means 

 the piston is driven to and fro with prodigious force, de- 

 veloping a power that is sufficient to work the heaviest ma- 

 chinery, and all by the simple pressure of the atmosphere 

 upon one side of the piston when the balancing resistance 

 on the other side is taken away. 



And yet this is only the pressure of one atmosphere. It 

 is precisely this amount of pressure which is exerted both 

 on the inside and on the outside of a glass bottle, or any 

 other receptable the interior of which has an open and free 

 communication with the outside air. 



If, however, this free communication is closed, and a 

 double quantity of air is forced into the receptacle 

 through a pipe fitted to it for the purpose, then we should 

 have the pressure of two atmospheres on the inside and 

 only one on the outside, and there would be a surplus 

 force of fifteen pounds upon every square inch of the in- 

 ternal surface, tending to burst the vessel. If treble the 

 quantity were introduced, then there would be a prepon- 

 derance of two atmospheres that is, of thirty pounds to 



