SOME CHARACTERISTICS OF THE AIR 



which moves up or down as the pressure increases 

 or decreases. A still larger movement is secured 

 in a compound barometer of this type, which is 

 constructed by joining six or eight of the metal 

 boxes (fig. 6). 



The mercurial barometer is quite a different 

 instrument. Indeed, at first it appears to have no 

 resemblance to the aneroid. A simple mercurial 

 barometer may be constructed in the following 

 manner: Secure a glass tube about three feet 

 long, with one end closed. Fill it with mercury, 

 close the open end with the finger, invert the 

 tube, and place the open end in a cup of mercury. 

 When the finger is removed the mercury in the 

 tube will not continue to fill the whole tube, but 

 will fall so that the top of the mercury column 

 is about 30 niches above the mercury in the 

 dish (fig. 7). If this column of mercury is ob- 

 served from time to time for several days, it 

 will be found to slowly rise or fall. The change 

 in height of the column is due to the change in 

 pressure of the air. 



6. Graduation of the barometer. In order to 

 be an efficient instrument, a barometer must be 

 graduated ; that is, a scale of some kind must 

 be attached to it in such a way that the move- 

 ments of the pointer or of the 

 top of the mercury column 

 may be easily read and re- 

 corded. This scale might be 

 marked in pounds pressure 

 per square inch, and this 

 would seem a very natural 

 thing to do, but it is not the 

 common method. Mercury 



FIG. 8. A standard 

 barometer 



A mercurial barometer 

 with thermometer and 

 scales. The graduations 

 allow both barometer 

 and thermometer to be 

 read. This is the stand- 

 ard form of barometer 

 used by the United 

 States Weather Bureau 



