f 



404 



THE 8TEAM-ENGINE. 



if a vessel, such as that represented in fig. 1, were filled with a fluid of this 

 kind, being open at the top, and not being restrained by any pressure incum- 

 bent upon it, the particles of the fluid would not rest in the vessel by their 

 gravity, as those of the liquid would do ; but they would, by their mutual 

 repulsion, fly asunder, and rise out of the vessel, as smoke is seen to rise from 

 a chimney, or steam from the spout of a kettle. Let us suppose, then, that 

 the vessel in which an elastic fluid is contained is closed on every side by 

 solid surfaces. In fact, let us imagine that the square or cubical vessel rep- 

 resented in fig. 1 is closed by a square lid at the top, A D, having contained 

 in it an elastic fluid, such as atmospheric air. 



If such a cover, or lid, had been placed upon a liquid, the cover would sus- 

 tain no pressure from the fluid, nor would any mechanical effect be produced, 

 save those already described in the case of the open vessel ; but when the 

 fluid contained in the vessel is elastic, as is the case with air, then the elas- 

 ticity (by which name is expressed the tendency of the particles of the fluid 

 to fly asunder) will produce peculiar mechanical effects, which have no exist- 

 ence whatever in the case of a liquid. 



It is true that, supposing the fluid to be air or any other gas or vapor, a 

 pressure will be produced upon the bottom, B C, of the vessel equivalent to 

 the weight of such fluid, and lateral pressures will be produced on the differ- 

 ent points of the sides by the weight of that part of the fluid which is above 

 these points ; but gases and vapors are bodies of such extreme levity, that 

 these effects due to their weight are neglected in practice. 



Putting, then, the weight of the air contained in the vessel out of the ques- 

 tion, let us consider the effect of its elasticity. If the vessel, as already de- 

 scribed, be supposed to contain atmospheric air in its ordinary state, the ten- 

 dency of the constituent particles to fly asunder will be such as to produce on 

 every square inch of the inner surface of the vessel a pressure amounting to 

 fifteen pounds ; this pressure being, as already stated, quite independent of the 

 weight of the air. In fact, this pressure would continue to exist if the air con- 

 tained in the vessel actually ceased to have weight by being removed from the 

 neighborhood of the earth, which is the cause of its gravity. 



Different gases are endowed with different degrees of elasticity, and the 

 same gas may have its elasticity increased or diminished, either by varying 

 the space within which it is confined, or by altering the temperature to which 

 it is exposed. 



If the space within which an elastic fluid is enclosed be enlarged, its elas- 

 ticity is found to diminish in the same proportion. Thus, if the air contained 

 in the vessel A B C D (fig. 1) be allowed to pass into a vessel of twice the 

 magnitude, the elasticity of the particles will cause them to repel each other, 

 so that the same quantity of air shall diffuse itself throughout the larger ves- 

 sel, assuming double its former bulk. Under such circumstances, the pressure 

 which it would exert upon the sides of the larger vessel would be only half 

 that which it had exerted on the sides of the smaller vessel. If, on the other 

 hand, it were forced into a vessel of half the magnitude of A B C D, as it 

 might be, then its elasticity would be double, and it would press on the inner 

 surface of that vessel with twice the force with which it pressed on that of 

 the vessel A B C D. 



This power of swelling and contracting its dimensions according to the 

 dimensions of the vessel in which it is confined, or to the force compressing 

 it, is a quality which results immediately from elasticity, and is consequently 

 one which is peculiar to the gases or elastic fluids, and does not at all apper- 

 tain to liquids. If the liquid contained in the vessel A B C D were trans- 

 ferred to a vessel of twice the magnitude, it would only occupy half the ca- 



