116 ELEMENTARY GENERAL SCIENCE CHAP. 



Pressure of the Atmosphere at Different Altitudes. 



Altitude ff tight of Pressure in 



Barometer. Ibs. per sq. in. 

 Sea-level 30 inches 15 



34 miles 15 ,, 7^ 



Boyle's Law. When the temperature remains the same, the 

 volume of a gas varies inversely as its pressure. 



Or, the temperature remaining the same, the product of the 

 pressure into the volume is constant. 



Boyle's Law applied to the Atmosphere. When the temperature 

 remains the same, the volume of a gas varies inversely as the pres- 

 sure. If a cubic foot of air at sea-level, where the pressure is 15 Ibs. 

 per square inch, were taken up to a height of 3| miles, where the 

 pressure is one-half, viz., 7^ Ibs. per square inch, it would expand to 

 two cubic feet. 



QUESTIONS ON CHAPTER VIII. 



1. How can it be proved that the air has weight? Illustrate your 

 answer by a few simple experiments. 



2. Given a glass tube 32 inches long, closed at one end, a bottle 

 of mercury (quicksilver), and a small cup. State how you proceed 

 (a) to construct a barometer, and (b) to show the readings of this 

 barometer. 



3. (a) State the average height of the mercury in a barometer at 



the sea-level and at the top of a mountain 3-i miles high. 

 (b) What is the cause of the difference in the height of the 



mercury column ? 

 (() What do you know concerning the condition of the upper 



layers of the atmosphere ? 



4. Describe the construction and use of an ordinary mercurial 

 barometer. 



5. How can the weight of air be determined ? In what way is 

 the pressure exercised by the atmosphere on the earth's surface in 

 consequence of its weight, stated ? How is it that we are able to 

 move about under the weight of the atmosphere ? 



6. Explain the chief reasons why mercury is the liquid usually 

 employed in the construction of barometers. 



7. State the principle on which the action of a mercurial bni ;;- 

 meter depends. Why is a water barometer longer than a mercurial 

 barometer ? What occupies the space above the mercurial column 

 in the latter instrument ? If a hole were bored through the glass 

 above the column of mercury, what would happen ? 



8. (a) Why does the mercury stand higher in the tube than in the 



cup of a barometer ? 

 (b) What is the average height of the mercury in a barometer 



tube at the sea -level ? 



(r) Why does the height vary from time to time ? 

 (d) Why is the barometer regarded as a " weather glass '' ? 



