110 ELEMENTARY GENERAL SCIENCE 



down until the top of the mercury in it is on a level with the 

 line. 



The student will now understand why it is so necessary to 

 remove all the air bubbles in Expt. 106. If this were not 

 done, when the tube was inverted the enclosed air would 

 rise through the mercury and take up a position in the top of 

 the 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 we can define as an instrument for measuring the 

 pressure exerted by the atmosphere. 



EXPT. 107. The preceding experiment will have shown you 

 that air pressing upon the surface of the mercury in the short 

 open arm of the (J'tube will balance a long column of mercury 

 in the closed arm. Slip a piece of india-rubber tubing upon 

 the open end and notice' what happens when you blow sharply 

 into it. Suck air out of the tube, and observe the result. 



These experiments show you the effect of increasing and de- 

 creasing the pressure upon the free surface of the mercury. 

 How will the height of the mercury column be effected (1) if the 

 pressure of the air decreases, (2) if the pressure of the air 

 increases ? 



Weight of Column of Atmosphere. The following is another 

 form of the experiment to show atmospheric pressure by means 

 of a barometer. 



EXPT. 108. 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. 



A column of mercury will be supported in the tube by the 

 pressure of the atmosphere. The distance between the top of 

 the column and the surface of the mercury in the cup will be 

 about 30 inches, or 76 cm. when the tube is vertical (Fig. 49, b). 

 If the tube is inclined so that the closed end of it is less than 



