HYDRGSTATICAL TABLES. 159 



when that steam is by any means condensed, cooled, 

 or reduced to water, that water will fail to the bottom 

 of the vessel ; and the cavity of the vessel will be al- 

 most u perfect vacuum. 



6. \Mienever a vacuum is made in any vessel, the air 

 by its weight will. endeavour to rush into the vessel, or 

 to drive in any other body that will give way to its 

 pressure ; as may be easily seen by a common syringe. 

 For, if you stop the bottom of a syringe, and then draw 

 up the piston, if it be so tight as to drive out all the air 

 before it, and leave a vacuum within the syringe, the 

 piston being let go will be driven down with a great 

 force. 



7. The force with which the piston is driven down, 

 when there is a vacuum under it, will be as the square 

 of a diameter of the bore in the syringe. That is to 

 say, it will be driven down with four times as much 

 force in a syringe of a two-inch bore, as in a syringe of 

 one inch : for the areas of circles are always as the 

 squares of their diameters. 



8. the pressure of the atmosphere being equal to 

 15 pounds upon a square inch, it will be almost equal 

 to 12 pounds upon a circular inch. So that, if the bore 

 of the syringe be round, and one inch in diameter, the 

 piston will be pressed down into it by a force nearly 

 equal to 12 pounds : but if the bore be two inches dia- 

 meter, the piston will be pressed down with four times 

 that force. 



And hence it is easy to find with what force the at- 

 mosphere presses upon any given number either of 

 square or circular inches. 



These being the principles upon which this engine is 

 constructed, we shall next describe the chief working 

 parts of it: which are, 1. A boiler. 2. A cylinder and 

 piston. 3. A beam or lever. 



The boiler is a large vessel made of iron or copper ; 

 and commonly so big as to contain about 2000 gallons. 



