PNEUMATICS. 



CHAPTER I. 



Division of Bodies Subject of Pneu* 

 matics* 



(1.) MATERIAL SUBSTANCES, in refer 

 ence to their mechanical properties, are 

 divided into solids said fluids. 



Wood, stone, metal in its ordinary 

 state, are instances of solids. One of 

 the most striking mechanical peculiari- 

 ties of this class is that, if to such a 

 body any force be applied, the whole 

 mass will be moved without suffering 

 any change in its figure or shape. 



Water, quicksilver, melted metal, air, 

 steam, are instances of fluids. This 

 class of bodies differ essentially in their 

 mechanical properties from the former. 

 That cohesion of parts which is the 

 cause of the preservation of the figure 

 of a solid, notwithstanding the applica- 

 tion of a force tending to change the 

 figure, has here no existence whatever. 

 The parts of a fluid are perfectly free to 

 move among each other, and immedi- 

 ately yield upon the application of the 

 smallest force. Fluids easily allow solid 

 bodies to pass through them, and their 

 surfaces always compose themselves 

 into a perfect level. On the other hand, 

 the cohesion of the parts of solids do 

 not permit the passage of another body 

 through them unless extraordinary 

 force be used, and their surfaces main- 

 tain any position with respect to a level 

 or horizontal plane in which they may 

 happen to be placed. 



(2.) Fluids are divided into two very 

 distinct classes, denominated, from their 

 characteristic mechanical properties, 

 elastic and inelastic. 



If a strong cylindrical vessel, of which 

 A B (Jig. 1 .) is a section, be filled to the 

 height C with water, and a piston or 

 plug D, accurately fitting the vessel, 

 and capable of moving water-tight in it, 

 be introduced over the surface of the 

 water, and a pressure be exerted on 



that surface by means of the piston, it 



will be found that no pres- * 



sure which can be produced * * 



will force the surface of the 



water lower in the vessel 



than its original height C. 

 Now let us suppose the B ;i 



water discharged from the 



vessel AB, and its place 



occupied by common air. 



Let the piston, as before, be 



introduced into the cylinder, 



fitting it so that no air can 



escape between the piston 



and the cylinder. A pres- 

 sure being now exerted on 



the piston it will immediately 



descend; but the moment AJ 



the pressure is removed, it 



will again ascend, and resume its first 



position. 



(3.) The property, in virtue of which 

 the water resisted the descent of the pis- 

 ton and would not admit of a diminished 

 bulk, is called incompressibility ; and, 

 on the other hand, the property by which 

 the air admitted the descent of the pis- 

 ton and was forced into a less bulk, is 

 called compressibility. Again, the pro- 

 perty manifested by the air in forcing 

 back the piston when the pressure was 

 removed, and resuming its original bulk, 

 is called elasticity. 



It appears, therefore, that elasticity 

 supposes compressibility, and a body 

 which is incompressible must necessa- 

 rily be also inelastic. Hence water is 

 an inelastic, air an elastic fluid. 



Strictly speaking, neither water, nor 

 any other liquid which we know of, is 

 perfectly incompressible or inelastic. It 

 was long supposed that water possessed 

 neither of these properties ; but accurate 

 experiments have shown, that it and 

 other liquids are both compressible and 

 elastic. A pressure of 15lbs. on each 

 square inch of surface, reduces the bulk 

 of water by one part in 21 740. The de- 

 grees of compressibility, however, which 

 B 



