PNEUMATICS. 



THE BAROMETER. 



In the Torricellian tube above described, we 

 have the well-known instrument called a barometer 

 (from two Greek words signifying ' weight ' and 

 'measure'). The weight of the atmosphere is 

 liable to fluctuations, and these are marked by the 

 rising and sinking of the top of the column at E. 

 The height of the column varies, at the level of the 

 sea, from 28 to 3 1 inches, the average height being 

 29-7. A scale is attached to the tube to mark 

 the rise and fall. It is important to observe, that 

 the height of the column is counted from the level 

 of the mercury in the open vessel, and that this 

 level is not steady. If the column sink, for 

 instance, part of the mercury in the tube descends 

 into the basin, and raises the level of the surface. 

 When the basin is comparatively wide, the rise is 

 insignificant, and is seldom attended to ; but 

 where great accuracy is required, either the scale 

 is so constructed as to allow for the alteration of 

 level, or the bottom of the basin is made flexible, 

 and can be raised up and down by a screw, so as 

 to bring the surface of the mercury always to the 

 same point on the outside of the tube. 



Barometers are of various constructions, accord- 

 ing to the uses they are intended for. The 

 common -weather-glass consists of a 

 glass tube, upwards of thirty inches 

 in length, closed at one end A, and 

 bent upwards at the open end C, as 

 represented in fig. 27. The height 

 of the column sustained is here to be 

 counted from E down to a point on 

 the tube on a level with F, the sur- 

 face of the fluid in the open end. 

 On the surface of F there floats a 

 small ball, from which a thread is 

 passed over a pulley G, and kept 

 stretched by a lighter ball W. When 

 the column at E sinks, the surface 

 at F rises, and carries up the float, 

 and the friction of the string turns 

 the pulley, and with it the index H, 

 whose motions are marked by a 

 graduated circle. Such instruments 

 Fig. 27. are not capable of any great ac- 

 curacy in ascertaining the actual 

 height of the column ; they indicate, however, 

 generally whether it is high or low, and whether 

 it is rising or falling, and these are the chief 

 points in prognosticating the weather. (See 

 METEOROLOGY.) 



ELASTICITY OF GASES. 



The laws which regulate the pressure of gases 

 are essentially the same as those already estab- 

 lished with regard to liquids. The fundamental 

 fact of transmitting pressure equally in all direc- 

 tions, is as true of the one class of substances as 

 of the other. But the effects in the case of gases 

 are much modified by their peculiar property of 

 elasticity, which requires careful consideration. 

 The elasticity of gases involves two things com- 

 pressibility and expansibility, and in the case of 

 common air, both properties are without any 

 known limit. 



Air unlimitedly Compressible. The compressi- 

 bility of air follows a remarkable law. Let ab (fig. 28) 

 be a cylinder 12 inches long, closed at b, and open 



at a, having an air-tight piston, d, moved up and 

 down by the rod c. Before the piston is inserted, 

 the air in the cylinder sustains the usual pressure 

 of the atmosphere, which is resting upon it nt 

 the open end a ; and if the area is an inch, the 

 amount of the pressure is 15 pounds. When the 

 piston is placed on the open end, and 

 about to enter, the direct action of the 

 outer air is cut off, but it continues to act 

 on the upper side of the piston, so that 

 the column inside still sustains 15 pounds. 

 Let the rod be then loaded with weights 

 till the piston descend through half the 

 cylinder, so as to squeeze the confined 

 air into half its original bulk ; it will be 

 found that 1 5 pounds have been laid on. 

 The piston is now forced down with a // 

 weight of 15 pounds, in addition to the 

 weight of the outer atmosphere ; the 

 pressure on the confined air is thus ex- f 

 actly doubled, and the effect is to compress 

 it into half\h& space. ./ 



If the weight is tripled by adding 15 

 pounds more, the piston descends two 

 inches farther to e, and the air is reduced ff 



to one-third of its original bulk. An b 



additional 1 5 pounds, making in all four Fig. 28. 

 atmospheres, sinks it to/, one-fourth from 

 the bottom ; and twelve atmospheres leave the 

 air confined between b and g, one-twelfth of the 

 whole space. This regular progression holds good, 

 without variation, for atmospheric air up to at 

 least twenty-seven atmospheres. 



It follows from this that the elastic force or 

 tension of compressed air that is, the force with 

 which it seeks to expand is exactly equal to the 

 compressing force, and inversely, as the space it 

 occupies. Atyj the air in the cylinder reacts upon 

 the piston with a force of four atmospheres ; at d, 

 where it occupies twice the space, it presses up 

 the piston with a force of only two atmospheres ; 

 at a, the under side of the piston is pressed with 

 a force of one atmosphere, so as exactly to balance 

 the weight of the external air. The law now ex- 

 plained was discovered independently by the two 

 philosophers Boyle and Mariotte, and is known 

 as Boyle's Law, or Mariettas Law. 



Air expansible without Limit. The spring of 

 air is not like that of a compressed feather or of a 

 piece of india-rubber, which loses its tension as 

 soon as it regains its original condition ; air can- 

 not be said to have any original volume, for it is 

 always striving to occupy a larger space. Suppose 

 an opening at the bottom of the cylinder (fig. 28), 

 and let the piston be at f; its upper and under 

 sides sustain each 15 pounds, for the external 

 atmosphere is admitted to both ; and air, like 

 water, presses equally in all directions, up as well 

 as down. If the opening is now closed by a plug 

 or a stop-cock, cutting off the external air from 

 acting on the under side, the piston does not 

 move, because the confined air presses on the 

 under side, not by its weight, but by its expansive 

 force, as much as the external air on the upper. 



Let the piston be next drawn upwards, and the 

 air not only follows it, but continues to press upon 

 the under side of it, though with a diminishing 

 force. When the piston has risen to d, the air 

 occupies twice its original bulk, and its tension or 

 pressure against the under side of the piston is 

 reduced to one-half, or 7^ pounds ; and if its 



