Page:The American Cyclopædia (1879) Volume XIII.djvu/648

 628 PNEUMATICS steep gradient. The smoke and bad air of or- dinary passenger tunnels traversed by locomo- tives are avoided, as the pneumatic carriage carries its own supply of pure air, and drives the foul air before it. PNEUMATICS (Gr. irvevpa, wind, air), that branch of general mechanics which treats of the equilibrium and motion of aeriform fluids. Many portions of this subject being embraced and treated under special topics, as AIE PUMP, ATMOSPHERE, BALLOON, BAEOMETER, BLOWING MACHINES, BOILING POINT, CHIMNEY, DIVING BELL, EXPANSION, EXPLOSIVES, FURNACE, GAS, HEAT, WIND, &c., a statement of the general theory only, with such applications as are not elsewhere made, will here be in place. Many gases, as the air, are permanent, preserving their gaseous form under all degrees of tem- perature or compression to which they have yet been subjected. Other gases, as chlorine and ammonia, by the agency of cold and pres- sure, change their state, become liquids or sol- ids, and for the time, of course, lose the pe- culiar properties of the aeriform condition ; these are non-permanent gases. As ordinarily understood, pneumatics treats of the action only of bodies in the form of permanent gases of which atmospheric air is the type ; but the principles of this science can be so extended as to investigate the elasticity and action of the vapors and non-permanent gases, through all stages of condensation, down to the liquid condition. Except when otherwise stated, the principles which follow will relate to the per- manent gases only. The distinguishing char- acters of these bodies grow out of the fact that their molecules do not sensibly cohere, but can move with perfect freedom both about and away from each other ; and that between these molecules there are repulsive forces greatly exceeding any forces of attraction which may act, causing them at all times to strive to recede from each other. From these circumstances the following principles are deduced: 1, all gases can be compressed, or if allowed will ex- pand, and so far as yet known, in the case of perfect gases, to an indefinite extent ; 2, when compressed, a perfect gas will always exert a pressure in the contrary direction, or against the compressing force, thus manifesting the peculiar form of elasticity possessed by these bodies, or what is called their expansive force, the measure or amount of which for a given case is termed the tension of the gas or vapor ; 3, wherever a gas or vapor is found to exist as a body, having appreciable density, this is the result of some confining pressure applied to it from without, and compelling its particles into a certain degree of proximity ; 4, when a body of gas does not expand, this is because the pressure from without equals and balances its tension at the time ; and 5, when a body of gas is at rest throughout all its parts, this is because at every point the various pressures exerted in different directions are in equilibrium. From the foregoing laws, and also from the fact that the particles of a gas possess weight as well as those of a liquid, the following laws also re- sult : 1, equal pressures ^n every direction are exerted upon and by every portion of a gaseous body at rest; 2, a pressure made on a confined body of gas, as in a liquid mass, is perfectly transmitted in every direction, and in the at- mosphere to great distances ; 3, such pressure is proportional to the area of surface receiving it, and consequently multiplied when the re- ceiving surface is larger than that communi- cating it ; 4, pressure on a given surface at a given depth, due to weight, is calculated in a similar way ; 5, the free surface of any such body, as the upper aerial surface, tends to a level at any place ; and 6, within any body of gas, at any given depth, there is exerted a supporting or buoyant power, which is as the density or tension of the gas at the place. The weight of a column of air resting on a horizontal square inch, at the sea level, is, at an average temperature, very nearly 14*6 Ibs. ; and a pressure of this amount is termed a pres- sure of one atmosphere. The first pneumatic law, discovered by Boyle in 1650, and inde- pendently by Mariotte in 1676, and known as Boyle's and Mariotte's law, affirms that, at a given temperature, the volume of an aeriform body at rest is inversely as the compressing force. Direct consequences are, that the den- sity and the tension are proportional to the compressing force. As the density of the air is about 7^-3- that of water, it follows from this law that if we could subject it to a pressure of 773 atmospheres, or about 11,320 Ibs. to the square inch, its density would equal that of water. Whether under such circumstances it would still remain a gas, is not known. The second great law of tension and pressure is that of Dalton and Gay-Lussac (1801), by both of whom it was independently discovered, ac- cording to which, when the tension of a gas or vapor is constant, the density diminishes as the increase of temperature ; in other words, for equal increments of temperature, a gas or per- fect vapor expands by the same fraction of its own bulk ; this being -^ of its volume at 32 F., and for each degree above that point, or about three eighths of its volume between 32 and 212. However long any permanent gas may be kept under pressure, its tension re- mains unimpaired. The laws of Mariotte and Dalton have been modified by the discovery that vapors and non-permanent gases undergo compression in a ratio greater than that of the increase of pressure upon them, and that near the point of liquefaction this deviation becomes very great. More recently, Mariotte's law has been found to need still further qual- ifications. Despretz (1829) announced that carbonic acid, ammonia, cyanogen, and some other gases, undergo at ordinary temperatures a compression more rapid than that of the increase of pressure, and in a ratio uniformly increasing; while above 14 atmospheres the result with hydrogen was the opposite. Re-