PLU VIAMETER

(Lat.pluvius, ram.) See Rain Gauge. PNEUMATICS. {Gr.^wjiJL»,ai^ or breath.) The science which treats of the mechanical properties of elastic fluids, and particularly of atmospheric air. Elastic fluids are divided into two cla es,— permanent gases, and vapours. The gases cannot be converted into the liquid state by any known proce of art; whereas the vapours are readily reduced to the liquid form by pre ure, or diminution of temperature. In respect of their mechanical properties there is, however, no e ential difference between the two cla es. Elastic fluids, in a state of equilibrium, are subject to the action of two forces; namely, gravity, and a molecular force acting from particle to particle. Gravity acts on the gases, in the same manner as on all other material substances; but the action of the molecular forces is altogether different from that which takes place among the elementary particles of solids and liquids; for, in the case of solid bodies, the molecules strongly attract each other (whence results their cohesion), and, in the case of liquids, exert a feeble or evanescent attraction, so as to be lud if-i ferent to internal motion; but, in the case of the g<ises,| the molecular forces are repulsive, and the molecules,! yielding to the action of these forces, tend ince antly to| recede from each other, and, in f;ict, do recede, until their] further separation is prevented by an exterior obstacleJ Thus, air confined within a close ve el exerts a constant! pre ure against the interior surface, which is not sensiblej only because it is balanced by the equal pre ure of thq atmosphere on the exterior surface. This pre ure exerted by the air against the sides of a ve el within whichj it is confined is called its elasticity, or clastic force, od tension. J [s. 953]