

PNEUMATICS. 



would result from an air too condensed, 

 man is not exposed to them by the ac- 

 tion of natural causes ; and it appears 

 that, in general, they are less than those 

 which are caused by the rarefaction of 

 the air. 



We need only cite here, as a proof of 

 the small magnitude of these inconveni- 

 ences, that which happens to divers, 

 when they have been shut up within a 

 bell which descended vertically in the 

 water, and in which the air, pressed by 

 the weight of the surrounding columns, 

 contracts itself more and more, in pro- 

 portion as the vessel is found at a greater 

 depth. The accidents which have oc- 

 curred to those who have continued for 

 a certain time under the bell, have 

 arisen in great part from the alteration 

 prodirced in the air by respiration, and 

 that which was most dangerous in this 

 fluid was the defect of renewing it. See 

 DIVING bell, BAROMETER, &c. 



The elasticity of the air is verified by 

 several well-known experiments. One 

 of the most ordinary is that in which we 

 employ the machine called the artificial 

 fountain. It consists of a metallic vessel of 

 a rounded form, its summit being pierced 

 with an orifice, through which the vessel 

 may be filled with water to about two-thirds 

 of its capacity. In this aperture a tube 

 is then fixed, which descends into the 

 vessel until it is within a little distance 

 of the bottom, while its upper part, 

 which projects from the orifice, is fur- 

 nished with a cock. To this same part 

 a forcing pump is adapted, and the cock 

 being opened, a great quantity of air is 

 injected into the vessel : this air, be- 

 ing lighter than water, rises above it, 

 and its elasticity augments with its den- 

 sity, in proportion as new strokes are 

 given to the piston. Then, after closing 

 the cock, the pump is removed, and a 

 kind of little hollow cone is substituted 

 for it, open at its summit, which is turn- 

 ed upwards ; as soon as the cock is 

 again opened, the condensed air exerts 

 its force upon the surface of the wa- 

 ter, and drives it through the canal that 

 is immersed into that liquid, whence it 

 is seen to shoot out, under the form of 

 a jet of more than twenty or thirty- 

 feet in height. An analogous effect 

 may be obtained, solely by diminishing 

 the natural elasticity of the air, by plac- 

 ing under the receiver of an air pump 

 a little vessel, in whicli all is similar to 

 what the artificial fountain presents at 

 the moment when the cock is opened 

 t.o give a free passage to the water, 



except that the air situated above this 

 liquid is in its ordinary stute. 



While the exhaustion is going on, the 

 air included in the vessel, and whose 

 pressure upon the water is no longer 

 balanced by that of the exterior air, 

 dilates itself, and gives birth to a jet 

 which rises under the receiver. (See 

 fig. 5.) But the most interesting experi- 

 ment relative to this object is that of 

 Boyle, and of Mariotte, to show that the 

 air contracts itself nearly in the ratio of the 

 weights with which it is pressed. These 

 kinds of experiments merit the prefer- 

 ence, since they are not confined to 

 merely proving the existence of a phe- 

 nomenon, but make known also how it 

 exists, by determining the law to wliich 

 it is subject. Take a glass tube a b (Plate 

 Pneumatics, fig. 1), bent into two 

 branches, the shortest of which is about 

 twelve inches high ; it must be equally 

 thick throughout, and hermetically seal- 

 ed at its extremity b. The other branch, 

 which is open at a, slttmld be at least 

 five feet, but if it were eight feet in 

 height, so much the better. The whole 

 is fixed upon a plate which carries di- 

 visions adapted to the two tubes. First, 

 let there be poured into the bent part 

 a little mercury, to obtain a line of level, 

 x Z, that we may estimate the number of 

 degrees comprised between that line and 

 the superior extremity of the shortest 

 branch. In this state of things the air 

 which occupies that branch maintains 

 an equilibrium, by its elasticity, with 

 the pressure of the column of atmo- 

 speric air gravitating in the other branch, 

 and whose pressure is transmitted by 

 means of the mercury comprised in the 

 inferior curvature. This pressure, as 

 we have seen in the article BAROMETER, 

 is equal to that of a mercurial column 

 of about twenty-nine or thirty inches iit 

 height. Afterwards, let mercury be 

 poured into the longest branch, and at 

 the same time the air in the other 

 branch will be condensed ; by the ex- 

 cess of the resulting pressure the mer- 

 cury will rise in the shorter branch un- 

 til an equilibrium is again produced. 

 Then measure, on one part, the length 

 of that column of compressed air, and 

 on the other the excess of the column of 

 mercury contained in the longest branch, 

 above that which occupies the shortest. 

 We will suppose, for more simplicity, 

 that this excess is equal to thirty inches ; 

 in that case, we shall find that the 

 column of compressed air is reduced to 

 the half of the height which it occupi- 



