282 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



the gas the mercury falls in the closed limb and rises 

 in the other. 



Spreiijfel's air pump procures a better vacuum 

 than the barrel pumps, though it takes a considerably 

 longer time. It consists of a funnel A, projecting, and 

 sealed into, a glass tube cd, not exceeding one-tenth of 

 an inch in diameter, and longer than 

 the barometer tube. The lower end 

 of the tube dips into an open glass 

 vessel B. A branch from the upper 

 part of the tube leads off to a receiver n, 

 which is to be exhausted. Mercury is 

 poured into the funnel, and falls from 

 it down the tube. In doing so it 

 carries air with it, drawn from the 

 receiver ; a series of short columns of 

 mercury, separated by air spaces, thus 

 move down the tube. The mercury being 

 Fig. 124. Spreu- caught in the open vessel below, soon 

 gel s Pump. covers the lower opening of the tube and 

 prevents air entering from below. As the exhaustion 

 becomes more and more complete the columns of mer- 

 cury become longer, and the air spaces less. At 

 length a regular column of mercury stands in the tube 

 to nearly the barometer height, and if mercury be now 

 allowed to fall from the funnel 011 to the mercury 

 column, no air is enclosed, and a hard metallic sound 

 is produced by the fall. 



The effects on the human body of atmo- 

 spheric pressure are various. On every square inch of 

 surface the pressure is 14*7 pounds. This pressure is 

 not felt because it is exercised in all directions, and 

 over all is, therefore, in equilibrium. It plays, never- 

 theless, a very important part in certain necessary 

 processes. The entrance of air into the lungs, and 

 exit from the lungs, are dependent on variations of 

 pressure. The cavity of the chest is air-tight, having 



