98 FIRST YEAR COURSE IN GENERAL SCIENCE 



downward pressure of the air which supports this column of 

 mercury must be equal to the weight of the column 

 that is, nearly fifteen pounds on a square inch. 



A complete barometer, which is to be carried from place 

 to place, is provided with a metal case to protect the 

 glass and to keep the tube upright. At the back of the 

 tube is a scale of inches, or centimeters, by which to read 

 the height of the mercury column. 



At sea level the world over, under ordinary conditions, 

 the height of the mercury in the tube of a barometer is 

 29.9 inches, or 76 centimeters, above the surface of the 

 mercury in the cup. In ascending to higher elevations, a 

 fall of one inch is observed during the first 900 feet, but 

 in an ascent of two miles the fall of mercury averages one 

 inch for every 1,000 feet of ascent. Leadville, Col., is 

 10,600 feet above sea level; its barometer reading under 

 ordinary conditions is about 20 inches. 



106. Differences in Air Pressure. If at sea level the 

 mercury in a barometer is higher than 76 centimeters, the 

 pressure of the air is greater than under ordinary conditions. 

 If the barometer reading is lower than 76 centimeters, the 

 pressure is below the normal. These differences in air pres- 

 sure are due to one, or both, of two causes: variation in the 

 amount of water vapor in the air, and variation in the tem- 

 perature. To illustrate: if the temperatures are the same, 

 a cubic foot of dry air has a greater density, and therefore a 

 greater pressure, than a cubic foot of moist air. If the 

 moisture is the same, a cubic foot of cold air has a greater 

 density, and therefore a greater pressure, than a cubic foot 

 of warm air. These two conditions may counteract each 

 other or they may tend to produce the same effect on the 

 barometer, sending the mercury up or down. 



107. Pressure upon our Bodies. We are not conscious 

 of the weight of the atmosphere because there is equal pres- 

 sure on all sides of the body. We are not crushed by this 



