STEAM. 



any excess would be attended with the effect of absorbing the 

 heat of combustion, without contributing to the maintenance of 

 that effect.* 



The mechanical force of steam is developed in three ways 

 I. By evaporation ; II. By expansion ; and III. By condensation. 

 We shall accordingly explain these severally. 



Pig i. 



7. I. POBCE DEVELOPED BY EVAPORATION. 



To render intelligible the manner in which a mechanical power 

 is developed in the conversion of water into steam, and the 

 circumstances which attend that remarkable 

 physical change, we will suppose a quantity of 

 pure water deposited in the bottom, A, of a 

 tube, B A, fig. 1. To render the explanation 

 more simple, we will suppose that the area of 

 the section of the tube is equal to a square 

 inch, and that the quantity of water deposited 

 in it is a cubic inch. We will further imagine 

 the tube to be glass, so that the phenomena 

 developed in it may be visible. Let a piston, 

 p, be imagined to be fitted in the tube, air 

 tight and steam tight, and to be placed in 

 immediate contact with the surface of the 

 water, so as to exclude all communication 

 between the water and the air above the piston. 

 In this case the piston would be pressed upon 

 the water by the pressure of the atmosphere 

 upon a square inch of surface added to the 

 weight of the piston itself. But the former 

 pressure is equal to 15lb.,f and therefore the 

 pressure on the surface of the water will exceed the weight of the 

 piston by 15lb. Now to simplify our explanation by excluding 

 all reference to the atmospheric pressure, and the particular 

 weight of the piston, P, we shall suppose both of these exactly 

 counterpoised by the weight, w, so that the piston shall be placed 

 in contact with the surface of the water, without, however, 

 exerting any pressure upon it. 



1 These conditions being understood, let a weight, say of 15 lb., 

 be placed upon the piston. P, and let a fire, a lamp or any other 

 regular source of heat, be applied to the bottom of the tube. If a 

 thermometer were immersed in the water under the piston, the 

 following effects would then be observed : 



See Tract on Fire. 



t See Tract on Air. 



198 



