162 



ATMOSPHERIC PRESSURE. 



the air it displaces. This principle is illustrated by the 

 baroscope, which consists of 

 a scale-beam supporting two 

 bodies of very unequal size (as 

 a hollow globe and a lead 

 ball), which balance one an- 

 other in the air. If the appa- 

 ratus thus balanced in the air 

 be placed under the receiver 

 of an air-pump, and the air 

 exhausted, the globe will de- 

 scend, thus seeming to be 

 heavier than the lead ball 

 which previously balanced it. 

 Is the globe actually heavier 

 than the lead, or not ? 



FIG. 97. 



EXERCISES. 



1. Give the pressure of the air upon a man the surface of whose 

 body is 14^ square feet. 



2. A soap-bubble has a diameter of 4 inches ; give the pressure 

 of the air upon it. (See Appendix A). 



3. What is the weight of the air in a room 30 by 20 by 10 feet ? 



4. What will be the total pressure of the atmosphere on a deci- 

 meter cube of wood when the barometer stands 760 mm. ? 



5. How much weight does a cubic foot of wood lose when weighed 

 in air ? 



6. (.) What is the pressure on the upper surface of a Saratoga 

 trunk 2 by 3-^ feet? (&.) How happens it that the owner can open 

 the trunk ? 



7. When -the barometer stands at 700 mm. what is the atmos 

 pheric pressure per sq. cm. of surface? Ans. 1033.6 g. 



Note. In round numbers, atmospheric pressure at the seale\el 

 is called 15 Ibs. to the sq. in., or 1 kilogram to the sq. cm. 



