ON PNEUMATIC EQUILIBRIUM. 207 



in the height, which will, therefore, be a little more than two miles and a 

 quarter. It may be found, by pursuing the calculation, that at the dis- 

 tance of the earth's semidiameter, or nearly 4000 miles above its surface, 

 the air, if it existed, would become so rare, that a cubic inch would 

 occupy a space equal to the sphere of Saturn's orbit : and on the other 

 hand, if there were a mine about 42 miles deep, the air would become as 

 dense as quicksilver at the bottom of it. 



It appears, therefore, that all bodies existing on or near the earth's 

 surface may be considered as subjected to the pressure of a column of air 

 28,000 feet high, supposing its density everywhere equal to that which it 

 possesses at the earth's surface, and which is usually such, that 100 wine 

 gallons weigh a pound avoirdupois, creating a pressure equal to that of 30 

 inches of mercury, or 34 feet of water, and which amounts to 14| pounds 

 for each square inch. This pressure acts in all directions on every 

 substance which is exposed to it : but being counterbalanced by the 

 natural elasticity of these substances, it produces in common no apparent 

 effects ; when, however, by means of the air pump, or otherwise, the pres- 

 sure of the air is removed from one side of a body while it continues to 

 act on the other, its operation becomes extremely evident. Thus, when two 

 hollow hemispheres, in contact with each other, are exhausted of air, they 

 are made to cohere with great force ; they are named Magdeburg hemis- 

 pheres, because Otto von Guerike, of Magdeburg,* constructed two such 

 hemispheres, of sufficient magnitude to withstand the draught of the em- 

 peror's six coach horses, pulling with all their force to separate them. By 

 a similar pressure, a thin square bottle may be crushed when it is suf- 

 ficiently exhausted, and a bladder may be torn with a loud noise : and 

 the hand being placed on the mouth of a vessel which is connected with the 

 air pump, it is fixed to it very forcibly, when the exhaustion is performed, 

 by the pressure of the air on the back of the hand ; the fluids also, which 

 circulate in the bloodvessels of the hand, are forced towards its lower sur- 

 face, and the effect which is called suction is produced in a very striking 

 manner. It is on the same principle that cupping glasses are employed, a 

 partial exhaustion being procured by means of the flame of tow, which 

 heats the air, and expels a great part of it : so that the remainder, when it 

 cools, is considerably rarefied. 



It was Galileo that first explained the nature of suction from the effects 

 of the pressure of the atmosphere ;t and his pupil Torricelli J confirmed his 

 doctrines by employing a column of mercury, of sufficient height to over- 

 come the whole pressure of the atmosphere, and to produce a vacuum in 

 the upper part of the tube or vessel containing it. In the operation of 



* Schotti, Mechanica Hydraulico-Pneumatica, 4to, 1657. Ottonis Guericke, 

 Experimenta Nova Magdeburgica, fol. Amst. 1670. 



f We may doubt whether this is not saying too much. Galileo proved that the 

 air has weight, and not, as was then believed, intrinsic levity. He actually weighed a 

 portion in a flask, but his determination of the specific gravity of air is not, as we 

 might conjecture, very accurate. Opere, iii. 47. The gravity of air was, however, 

 knoton to the ancients. See Aristotle, De Coelo, lib. iv. 



J On this subject, see Pascal, Traite de rEquilibre des Liqueurs, Par. 1669, p. 177. 

 Cartesii Opera, ii. 243, 246 ; andMontucla Histoire des Mathematiques, ii. 203. 



