SECT, vi.] CONDENSING ENGINES. 185 



When the piston arrives near to the bottom of the cylinder, the passage to the 

 condenser is shut, and the valve in the piston opens ; then the steam above the 

 piston passes through to below it, as the piston rises by the action of the counter 

 weight ; and, being at the top, the valve in the piston closes, and the valve to the 

 condenser opens, and another stroke is made, and so on successively. 



But since a large quantity of water is used at each stroke, and the water con- 

 tains a considerable quantity of air, the condenser would soon become filled with 

 air and water, and the engine would cease to work ; to avoid this the air pump is 

 added, which, being worked by the beam, makes a stroke at each stroke of the 

 steam piston, and clears the condenser of air and water. 



387. In an atmospheric engine with a condenser, the principal difference 

 consists in the steam being let both into and out of the cylinder by passages at the 

 bottom ; and the descent of the piston is caused by the pressure of the atmosphere 

 on its upper surface, which is open to the air. 



388. But in the atmospheric engine, as constructed before Mr. Watt's improve- 

 ment of condensing in a separate vessel, the jet of cold water was thrown into the 

 cylinder itself at each stroke ; and hence the cylinder required to be heated and 

 cooled at each stroke at a great expense both of fuel and cold water. 



The addition of a separate condenser was the most valuable of Mr. Watt's 

 improvements ; his next in importance was the double acting engine. The saving 

 from the concentration of power, which results from these improvements, can be 

 judged of only by those who are intimately acquainted with the employment of 

 mechanical power. To them the merit of his invention must be known and duly 

 appreciated, and by their estimation it must ultimately be valued in public 

 opinion. 



389. The double acting engine, in general construction, resembles the single 

 one described in the preceding article. (Art. 386.) It differs in having a passage 

 from the boiler both to the top and the bottom of the cylinder, and a similar 

 passage from both to the condenser. Hence, it does not require a counter weight 

 to raise the piston, nor that the steam should pass from the upper to the lower 

 side. The force of the steam impels the piston in both directions ; and compared 

 with a single engine of the same size, a double quantity of steam is used, and 

 double power is exerted in the same space and time. 



390. In any of these species, steam may act expansively, whether the 

 atmospheric or steam pressure be used ; but the moving force may be rendered 

 more uniform, by using two cylinders of different sizes : in the smaller of these 

 cylinders the steam acts with all its force throughout the stroke, and in the other 

 it gradually expands as the stroke proceeds, and therefore the moving force is 

 variable : but as the forces on both pistons jointly constitute the moving force, 



2A 



