184 THE WEATHER 



speak of the pressure as being so many centimeters or inches, 

 meaning thereby that the atmospheric pressure supports a 

 column of mercury that number of centimeters or inches 

 in height. Another way of expressing the pressure is in 

 pounds per square inch, or in grams per square centimeter. 



Exercise 48. To Measure the Atmospheric Pressure 



Fill a Torricellian tube, getting the mercury free of air. Measure 

 the height of the column; then raise the tube till its open end is just 

 below the surface of mercury in the cup; slip your finger over the open 

 end and raise it from the cup. Being careful not to lose any of the 

 mercury, empty the tube into a beaker. Weigh the beaker and mer- 

 cury and record the weight. Pour the mercury into a small measur- 

 ing graduate. Weigh the empty beaker. Record the volume of mer- 

 cury in the graduate. Now calculate the weight, or downward pres- 



i? A- f 11 Volume . 



sure, of mercury per square centimeter as follows: -. . , = Area 



Weight of mercury _^ . 



of tube. r = .Pressure per square unit area. 



A student obtained the following results: 



Height of mercury 74 . 6 cm. 



Weight of beaker and mercury 113.7 grams 



Weight of beaker 12 . 2 grams 



, Therefore, weight of mercury 101 . 5 grams 



Volume of mercury 7.46 c.c. 



By calculation, area of tube 0. 10 sq. cm. 



And, by calculation, pressure per sq. cm. 1015 grams 

 Prove the correctness of his calculations. 



At sea level the average atmospheric pressure is about 

 1030 grams per sq. cm., or about 14.7 Ib. per sq. in. 



1. A schoolroom is 30 ft. square and 13 ft. high. What is the 

 weight of the air in the room? What is the amount of the air pres- 

 sure upon the floor of the room? Upon each of its four sides? 

 Upon its ceiling? 



2. Measure the dimensions of your schoolroom. Compute the 

 weight of air in it. Compute the pressure upon its floor ; upon each 

 of its walls; upon its ceiling. 



