vnr ATMOSPHERIC PRESSURE AND BOYLE'S LAW 109 



tube with its closed end downwards and pour mercury into 

 it (being careful to remove all air bubbles) until the liquid 

 reaches the short tube. Then fix the arrange- 

 ment upright as in Fig. 48. 



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 thirty inches. 



The instrument used in Expt. 106 is evidently 

 similar to a U-tube. Referring to Fig. 48, it is 

 clear that there is a column of mercury supported 

 by some means which is not at first apparent, or 

 else the mercury would sink to the same level in 

 the long and the short tube, for we know that 

 liquids find their own level. If a hole were made 

 iiT the closed end of the tube this would im- 

 mediately happen. There will be no difficulty 

 from what has been already said, in understand- 

 ing that the column of mercury is kept in its posi- 

 tion 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. 

 48, the mercury column to its upper limit in the 

 long tube, the air to its upper limit, which, as will 

 be seen, is a great distance from the surface of the 

 earth. If for any reason the weight of the atmo- 

 sphere 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 supported 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 arrangement shown in the accompanying illustration, a line 

 is drawn at a fixed point O, and the short tube is shifted up or 



Fir,. 48. To ex- 

 plain the Prin- 

 ciple of the 

 Barometer. 



