134 



A YEAR IN SCIENCE 



the tubes. The shortness of the mercury tube compared 

 with that of water makes the use of mercury more 

 convenient for both experimental and practical pur- 

 poses. 



Variations in pressure due to elevation. The pressure 

 of the atmosphere upon any square inch of surface 

 depends upon the total quantity of air directly over- 

 lying that surface. The quantity will be greater at 

 the level of the sea than on the summit of a high 

 mountain, in the bottom of a valley than on top of a 

 hill, at the base of a building than in one of its upper 

 stories. 



Other factors determine the pressure as well as ele- 

 vation. Condition of the weather, temperature, winds, 

 amount of water vapor, all tend to modify the atmos- 

 pheric pressure not only from day to day, but even 

 from hour to hour. It is possible to record these 

 changes in pressure. The instrument 

 used for this is called a barometer. 



Barometer. A glass tube with 

 thick walls, closed at one end, is 

 filled with mercury. This is then 

 inverted into a small dish of mer- 

 cury. The whole is attached to a 

 board which is marked in inches or 

 centimeters. As the pressure of air 

 upon the surface of the mercury in 

 the dish varies, the level of the liquid 

 Fia blmte imple ^ the tube rises or falls. 



