62 



JUNIOR GRADE SCIENCE 



Pressure exerted by the atmosphere. It has been seen that air 

 has weight, and as there is a very considerable quantity of it, ex- 

 tending as it does to well over 100 miles from the surface of the earth, 



it may be expected to exert a pressure 

 on bodies beneath it. That this is the 

 case is shown by placing water in a 

 U-tube and sucking the air out of one 

 limb : the water rises in that limb, 

 because the air on the water in the 

 open limb forces it down, the air in the 

 other limb which previously counter- 

 balanced this pressure being removed 

 by suction. 



The same kind of effect is noticed when 

 ah* is sucked out of a Hare's apparatus, 

 one limb being inverted over water and 

 the other over mercury. In this case 

 the water rises further than the mercury, 

 and measurements of the lengths of the 

 respective columns show that their 

 lengths are inversely proportional to 

 their densities, so that per unit of cross- 

 section the same weight is supported in 



each column. The surface of the liquid 

 correspondg 



FIG. 49. Hare's apparatus for com- 

 paring the densities of liquids. 



to the surface of the liquid in the open limb of the U-tube, and the air 

 pressing down on it supports the liquid hi the limb. 



26A. MEASUREMENT OF THE PRESSURE OF THE AIR. 



i. The principle of the mercurial barometer. Procure a barometer tube 

 and fit a short piece of india-rubber tubing upon its open end. Tie the 

 free end of the tubing to a glass tube about six inches long, open at both 

 ends. Rest the barometer tube with its closed end downwards and pour 

 mercury into it (oeing careful to remove all air bubbles) until the liquid 

 reaches the short tube. Then fix the arrangement upright as in Fig. 50. 

 The mercury in the long tube will be seen to fall so as to leave a space of 

 a few inches between it and the closed end. The distance between the top 

 of the mercury column in the closed tube and the surface of that in the 

 open tube will be found to be about 30 inches. 



ii. The cistern barometer. Procure a thick glass tube about 36 inches 

 long and closed at one end. Fill the tube with mercury; place your 

 thumb over the open end ; invert the tube ; place the open end in a cup 

 of mercury and take away your thumb (Fig. 51). Measure the distance 



