PNEUMATICS. 



tfer 55 : this ratio must vary in propor- 

 tion to the changes in the height of the 

 barometer, and it varies also ^'^th part 

 for every degree of the thermometer 

 above or below temperature : hence the 

 cubic foot of air, of water, and of quick- 

 silver, may be taken as I.J. ounce, 1000 

 ounces, and 13,600 ounces. 



The gravity of the air being onse 

 known, it should seem that it could not 

 be difficult to infer that the ascent of 

 water, in the body of a pump, must be 

 occasioned by the pressure of that fluid. 

 This, however, was not the case : Galileo 

 had no notion of it. 



Some ItaJian conduit-makers being ask- 

 ed if they would construct sucking-pumps, 

 whose tubes should be more than 33 feet 

 in height, remarked, with surprise, that 

 the water refused to rise above that 

 limit. They requested of Galileo the 

 explication of this singular fact ; and it is 

 affirmed that the philosopher,being taken 

 unawares, replied, that nature did not 

 entertain the horror of a vacuum beyond 

 S3 feet. Torricelli, a disciple of Galileo, 

 having meditated upon this phenomenon, 

 conjectured that water is elevaied in 

 pumps by the pressure of the exterior 

 air; and that this pressure has only 

 the degree of force necessary to coun- 

 terbalance the weight of a column of 

 water of 33 feet. He verified Uiis con- 

 jecture by an experiment, for which na- 

 tural philosophy owes him a double obli- 

 gation, since it serves to render evident 

 an important discovery, while it has pro- 

 cured us the barometer. Torricelli saw 

 the mercury stand 29 or 30 indies in a 

 glass tube, sealed at its upper part, and 

 situated vertically ; and the height thus 

 under consideration being to that of 33 

 feet in the inverse ratio of the densities of 

 water and of mercury ; he concluded that 

 the phenomenon belonged to statics, and 

 that it was really, as he had conjectured, 

 the pressure of the air which caused wa- 

 ter or mercury to rise until an equilibri- 

 um was produced : this occurred in 1643. 

 The year following, the news ofTorri- 

 celli's experiment was disseminated in 

 France by a letter written from Italy to 

 Father Mersenne. The experiment was 

 performed again in 1646, by Mersenne 

 and Pascal ; and the latter devised, in 

 1647, a method of rendering it still more 

 decisive, by making it at different alti- 

 tudes. He invited, in consequence, his 

 friend Perrier to repeat the experiment 

 upon the mountain Puy-de-Dome, and to 

 observe whether the column of mercury 



would descend in the tube in proportion 

 as it became more elevated. We may 

 see from the letter of Pascal to Perrier, 

 where he seems to avoid the name of 

 Torricelli, that he had not yet entirely 

 renounced the chimera of the horror at a 

 vacuum which was attributed to nafure, 

 and that by admitting that this horror was 

 not invincible, he was not bold enough to 

 assert that it never obtained. The suc- 

 cess of the experiment completely re- 

 moved the delusion. Yet this experi- 

 ment was only a confirmation of that by 

 Torricelli, and therefore yielded an addi- 

 tional ray to the stream of light which 

 issued from it. The pressure of the at- 

 mosphere, upon a given surface, being 

 nearly the same as would be exerted up- 

 on that surface by a column of water of 

 33 feet high ; from this datum lias been 

 computed the effect of the pressure un- 

 der consideration, with respect to a man 

 of medium magnitude, and it has been 

 found that it is equivalent to a weight of 

 about 33,600 pounds. Considerable as 

 this weight is, its pressure is exerted un- 

 known to us, because it is continually 

 balanced by the re-action of the elastic 

 Huids comprised in the interior cavities of 

 our bodies ; and though the air is subject 

 to continual variations, which augment 

 or diminish its density, in consequence 

 of clxanges of temperature, and of the ac- 

 tion of different natural causes, yet as 

 these variations are generally confined 

 within narrow limits, and succeed each 

 other with comparative tardiness, they 

 do not affect us commonly, except in a 

 manner scarcely perceptible. But if 

 there happen a sudden change, as when 

 a man is raised to great heights, the 

 rupture of the equilibrium which ensues, 

 has a very marked influence upon the 

 animal economy. He then experiences 

 an extreme i'atigue, and absolute inabili- 

 ty to continue his progress ; a drowsiness 

 under which he sinks in spite of himself ; 

 the respiration becomes thick and difli 

 cult ; the pulsations take an accelerated 

 motion. To explain these effects, H 

 must be considered that the state ot" 

 well-being, in all that depends upon re- 

 spiration, requires that a determinate 

 quantity of air should pass through the 

 lungs in a given time. If, therefore, 

 the air that we respire becomes much 

 more rare, the inspirations must of ne- 

 cessity be proportionally more frequent ; 

 which will render the respiration more 

 difficult, and will occasion the various 

 symptoms to which we have referred. 

 Yvith regard to the inconveniences that 



