PNEUMATICS. 175 



so that the external pressure is met and counterbalanced by the internal re- 

 sistance. 



If a man, or animal were at once relieved of all atmospheric pressure, all 

 the blood and fluids of the body would be forced by expansion to the surface, 

 and the vessels would burst 



,„, „ , . Persons who ascend to the summits of very high mountains, 



What effect is , . ,..,„, • ^ 



experienced in or who rise to a great elevation m a balloon, have expenenced 



rising to great ^.j^g most intense sufFeriner from a diminution of the atmos- 

 elevatioas ? ° 



pheric pressure. The air contained in the vessels of the bodj; 



being relieved in a degree of the external pressure, expands, causing intense 

 pain in the eyes and ears, and the minute veins of the body to swell and 

 open. Travelers, in ascending the high mountains of South America, have 

 noticed the blood to gush from the pores of the body, and the skin in many 

 places to crack and burst. 



• We become painfully sensible of the effect of withdrawing 



principle of the external pressure of the atmospb.ere from a portion of the 

 " c"PPi"S • skin of the body in the operation of cupping. This is efiected 



in the following manner : a vessel with an open mouth is connected with a 

 pump, or apparatus for exhausting the air. The mouth of the vessel is ap- 

 plied in air-tight contact with the skin ; and by working the pump a part of 

 the air is withdrawn from the vessel, and consequently the skin within the 

 vessel is reUeved from its pressure. All other parts of the body being still 

 subjected to the atmospheric pressure, and the elastic force of the fluids con- 

 tained in the body having an equal degree of tension, that part of the skin 

 which is thus reheved from the pressure swells out, and will have the ap- 

 pearance of being sucked into the cupping-glass. 



If the lips be applied to the back of the hand, and the breath drawn in so 

 as to produce a partial vacuum in the mouth, the skin will be drawn, or sucked 

 in — not from any force resident in the lips or the mouth drawing the skin in, 

 but from the fact that the usual external pressure of air is removed, and the 

 pressure from within the skin is allowed to prevail 



The sense of oppression and lassitude experienced in sum- 

 ten feel op- mer previous to a storm, is caused by v -ica 

 S'^storat'^^"" a diminished pressure of the atmosphere. ^^^- ^^^ 



The external air, in such instances, be- 

 comes greatly rarefied by extreme heat and by the con- 

 densation of vapor, and the air inside us (seeking to 

 become of the same rarity) produces an oppressive and 

 suffocating feeUng. 



„ . ^ 387. The direct effects of atmospheric 



Describe tne , .,, , , 



common suck- pressure may be illustrated by many 



*'■• practical experiments. If a piece of 



moist leather, called a sucker, Fig. 1G4, be placed in 

 close contact with any heavy body, such as a stone, or a 

 piece of metal, it will adhere to it, and if a cord be at- 

 tached to the leather, the stone, or metal, may be raised 



