508 SCIENCE OF HOME AND COMMUNITY 



periments were performed in a previous chapter to show that 

 air has weight. This fact has not been known for very many 

 years. 



Torricelli's experiment. Nearly three hundred years ago, 

 a man named Torricelli first performed an experiment which 

 showed that air exerts pressure. This experiment has be- 

 come historic and has been performed many times since. 

 We will perform it now because it will help us better to 

 understand the principle of the barom- 

 eter by which air pressure is measured. 

 We shall need a dish of mercury and 

 a glass tube about three feet long, 

 closed at one end. The tube is filled 

 with quicksilver, the thumb placed over 

 the end, and the tube inverted in the 

 dish of mercury. The thumb is re- 

 moved after the open end is under the 

 surface of the mercury. The mercury 

 in the tube falls about six inches and 

 then stops. It is held up by the pres- 

 sure of air exerted on the surface of the 



FIG. 201. Torricelli's mercury. 



experiment. The barometer. This shows the prin- 



ciple used in the construction of barometers. The height 

 of mercury is a standard by which to measure the pressure 

 of air. The weight of this column of mercury is just equal 

 to the weight of a column of air of the same diameter and 

 extending up as far as the air goes, which is fifty miles or 

 so. So that a column of air fifty miles high is equal to 

 the weight of a column of mercury about thirty inches high. 

 Figure 202 shows an ordinary mercurial barometer. It is 

 built on the principle of the apparatus used in Torricelli's 

 experiment. The tube above the mercury contains a vacuum; 

 there is no air there to prevent the mercury from moving uj 

 the tube. 



