THE STEAM-ENGINE. 



403 



Fig. l. 



the amount of which would be equal to the weight of the metallic mass. No 

 pressure would be exerted against the sides ; for the coherence of the parti- 

 cles of the solid maintaining them in their position, the removal of the sides 

 would not subject the solid body contained in the vessel to any change. 



Now let us suppose this solid mass of lead to be rendered liquid by being 

 melted. The constituent particles will then be deprived of that cohesion by 

 which they were held together ; they will accordingly have a tendency to sep- 

 arate, and fall asunder by their gravity, and will only be prevented from ac- 

 tually doing so by the support afforded to them by the sides, A B, D C, of the 

 vessel. They will therefore produ .e a pressure against the sides, which was 

 not produced by the lead in its solid state. This pressure will vary at differ- 

 ent depths : thus a part of the side of the vessel at P will receive a pressure c 

 proportional to the depth of the point P below the surface of the lead. If, for S 

 example, we take a square inch of the inner surface of the side of the vessel 

 at P, it will sustain an outward pressure equal to the weight of a column of 

 lead having a square inch for its base, and a height equal to P A. And, in 

 like manner, every square inch of the sides of the vessel will sustain an out- 

 ward pressure equal to the weight of a column of lead having a square inch 

 for its base, and a height equal to the depth of the point below the surface of 

 the lead. 



We have here proceeded upon the supposition that no force acts on the up- 

 per surface, A D, of the lead. If any force presses A D downward, that force 

 would be transferred to the bottom by the lead, and would produce a pressure 

 on the bottom B C, equal to its own amount, in addition to the weight of the 

 lead ; and if the lead were solid, this would be the only additional mechanical 

 effect which such a force acting on the surface, A D, of the lead would pro- 

 duce. But if, on the other hand, the lead were liquefied, then the force now 

 adverted to, acting on the surface, A D, would not only produce a pressure 

 on the bottom B C, equal to its own amount in addition to the weight of the 

 lead, but it would also produce a pressure against every part of the sides of 

 the vessel, equal to that which it would produce upon an equal magnitude of 

 the surface A D. 



Thus, if we suppose any mechanical cause producing a pressure on the sur- 

 face A D amounting to ten pounds on each square inch, the effect which would 

 be produced, if the lead were solid, would be an additional pressure on the 

 base B C amounting to ten pounds per square inch. But if the lead were 

 liquid, besides this pressure on each square inch of the base B C, there would 

 likewise be a pressure of ten pounds on every square inch of the sides of the 

 vessel. 



All that has been here stated with respect to a square or a cubical vessel, 

 will be equally applicable to a vessel of any other form. 



The second class of fluids are distinguished from liquids by the particles 

 not merely being destitute of cohesion, but having a tendency directly the 

 reverse, to repel each other, and fly asunder with more or less force. Thus, 



