*274 lECTURE xxir. 



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, tlie pressure 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 hemis- 

 pheres, of sufficient magnitude to withstand the draught of the emperor's six 

 coach horses, pulling with all their force to separate them. By a similar pres- 

 sure,athin square bottle may be crushed when it is sufficiently 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 surface, 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; and his pupil Torricelli confirmed his 

 doctrines by employing a column of mercury, of sufficient height to overcome 

 £he 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 sueking up a 

 fluid through a pipe, with the mouth or otherwise, the pressure of the air is 

 but partially removed from the upper surface of the fluid, and it becomes ca- 

 pable of ascending to a height which is determined by the difference of the 

 densities of the air within and without the cavity concerned: thus, an ex- 

 haustion of one fourth of the air of the cavity would enable us to raise water 

 to the height of 84- feet, and mercury to 7i inches, above the level of the re- 



