182 THE WEATHER 



receiver. Do the palm glass and stand also rise with your hand? 

 Are you really lifting the glass and stand in the sense that "lift" 

 usually has? Just what does your muscular effort overcome? What 

 force is it then, that holds the glass against your hand? 



(d) If you are not thoughtful you will draw the wrong conclusion 

 from experiment (6). The hand was not drawn into the palm glass, 

 neither was there any extra pressure applied to the back of the hand. 

 Was there blood pressure in the arteries and the capillaries lying 

 just beneath the skin of the back of your hand? Was there blood 

 pressure in the arteries and capillaries lying just above the skin of 

 the palm of your hand? What, then, was the source of the pres- 

 sure which forced the skin of the palm of your hand down into the 

 opening of the palm glass? 



(e) Tie the rubber dam over the mouth of a thistletube. Attach 

 a rubber tube to the stem of the thistletube. Suck some of the air 

 from the tube and note how the dam is pressed into the tube's mouth. 

 Pinch the tube so that air cannot enter; then hold the thistletube 

 with its mouth upward, downward, sidewise. May we conclude that 

 air pressure is equal in every direction? 



When we consider that the pressure upon the inner side of 

 the skin of the palm of the hand became almost painful when 

 the air was pumped from the palm glass, and when we are 

 told that the total air pressure upon our bodies is many 

 hundreds of pounds, we may wonder why we ordinarily feel 

 no inconvenience from this great pressure. It is because 

 there is the same pressure, so far as the atmosphere is con- 

 cerned, exerted on every side of our bodies, both outside and 

 inside. The air enters our lungs; our blood is therefore al- 

 ways under a somewhat greater pressure than the surface of 

 our bodies. 1 



210. Torricelli's Experiment. Galileo, who proved that 

 air has weight, observed that a pump in a deep well raised 

 the water only about 32 ft. All scientists before him had 



1 Blood pressure usually increases with age. With children and young 

 people it usually amounts to about 2 Ib. to the sq. in. more than atmospheric 

 pressure ; with old people it usually amounts to 3 Ib. or more. In some cases, 

 even in middle life, it amounts to 3 or even 4 Ib. Usually, abnormally high 

 blood pressure indicates ill health, premature old age, and short life. Life 

 insurance companies and physicians depend largely upon blood pressure to 

 determine the probable length of life and general health of a person. 



