THE BAROMETER 129 



pressure of the atmosphere at any place on the earth's 

 surface is liable to change. Since measurement of atmos- 

 pheric pressure is of great importance in the 

 study of atmospheric conditions, it is necessary 

 to have an instrument by which changes in pres- 

 sure can be readily measured. An instrument 

 designed for this purpose is called a barometer. 

 There are two kinds of barometers in common 

 use, the mercurial and the aneroid. 



If the tube used in Torricelli's Experiment 

 (page 116) is fixed in an upright position, and the 1C 



height of the mercury marked from time to time, 

 it will be found that the height of the mercury 

 column changes slightly, thus indicating greater 

 or less atmospheric pressure. In Torricelli's Ex- 

 periment, therefore, we had a mercurial barometer 

 in rough form. 



The best form of this instrument consists of 

 a glass tube of uniform bore about eighty centi- 

 meters long and closed at one end. After being 

 carefully filled with pure mercury, it is inverted 

 in a cistern of mercury. The cistern of mercury 

 has a sliding bottom easily moved up and down 

 by means of a set screw. At the top of the 

 cistern there is a short ivory peg. The lower 

 end of the ivory peg is at an exactly measured 

 distance from the bottom of a scale. The scale 

 is placed beside a slit near the top of a metallic 

 tube which is firmly fastened to the cistern and 

 surrounds and protects the glass tube. 



When it is desired to read the barometer, the , 



MERCURIAL 



sliding bottom of the cistern is raised or lowered BAROMETER 



