64 JUNIOR GRADE SCIENCE 



column of air many miles high balances a column of mercury about 

 30 inches high. 



If, however, a hole were made in the closed end of the tube the 

 balance would be disturbed, and the mercury would fall down the 

 long tube and flow out of the short tube O. 



The column of mercury is kept in its position by the weight of the 

 atmosphere pressing upon the surface of the mercury in the short 

 open tube. The weight of the column of mercury and the weight of 

 a column of the atmosphere with the same sectional area is exactly 

 the same ; both being measured from the level of the mercury in the 

 short stem of the apparatus shown in Fig. 50, the mercury column to 

 the top of the column in the long tube the air to its upper limit, which, 

 as has been seen, is a great distance from the surface of the earth. If 

 for any reason the weight of the atmosphere becomes greater, the 

 mercury will be pushed higher to preserve the balance ; if it should 

 become less, then similarly the amount of mercury which can be sup- 

 ported will be less, and so the height of the column of mercury is 

 diminished. 



The height must in every case be measured above the level of the 

 mercury in the tube or cistern open to the atmosphere. In the usual 

 arrangement, illustrated by Fig. 50, a line is drawn at a fixed point O, 

 and the short tube is shifted up or down until the top of the mercury 

 in it is on a level with this line. 



It will now be understood why it is so necessary to remove all 

 the air bubbles in Experiment 26A, i. If this is not done, when the 

 tube is inverted the enclosed air would rise through the mercury and 

 take up a position in the top of the longer tube, above the mercury. 

 The reading would not then be thirty inches, for instead of measuring 

 the whole pressure of the atmosphere, what we should really be measuring 

 would be the difference between the pressure of the whole atmosphere 

 and that of the air enclosed in the tube. In a properly constructed 

 barometer, therefore, there is nothing above the mercury in the tube 

 except a little mercury vapour. 



An arrangement like that described constitutes a barometer, which may 

 be denned as an instrument for measuring the pressure exerted by the 

 atmosphere. 



The cistern barometer. Other forms of barometer are often em- 

 ployed for the determination of the pressure of air. A very common 

 arrangement is that of Experiment 26A, ii., which is a repetition of one 

 by an Italian physicist, Torricelli. The principle of its action is precisely 

 that of the barometer iust described, except that the U-tube principle 

 is not immediately apparent. There is, however, the same balance 



