106 FIRST YEAR COURSE IN GENERAL SCIENCE 



air, as it enters the chamber faster than it can go out. 

 While the piston is lifted for another stroke, the elastic air 

 expands and forces out the remaining water, thus giving a 

 moderately steady stream. 



In parts of the country where there are no elevated 

 regions, many towns and cities have their water supply in 

 lakes or rivers lower than some sections of the town. 

 Such places have a pumping station where machine-power 

 pumps work constantly to fill elevated reservoirs or stand- 

 pipes and privately owned tanks. From these the water 

 is delivered under pressure to buildings and fire hydrants. 

 Both lift pumps and force pumps may be worked by wind- 

 mills or gas engines. 



EXERCISES 



1. Explain the difficulty of filling an " empty " bottle held in a 

 stream of water. 



2. Why is a bottle still full of air when some of it has been removed? 



3. Why is there physical discomfort at high elevations? 



4. In an inverted closed tube, water is supported by atmospheric 

 pressure to a height of 34 ft.; mercury, to a height of 30 in. Why is 

 the mercury column shorter? What is the relation of the density of 

 mercury to that of water, as shown by this fact? 



6. A vertical open tube 40 in. long is placed with its lower end in 

 a cup of mercury, and its upper end is connected with an air pump. 

 If some of the air is removed from the tube, what will happen? Give 

 the reason. 



6. What adjective is commonly applied to air under more than 

 ordinary pressure? How is it used? 



7. A mass of gas under ordinary atmospheric pressure occupies 

 3,000 cu. cm. How many cubic centimeters will there be, if the pressure 

 is doubled? If halved? 



8. Will a balloon filled with gas expand or contract as it rises? 



9. Explain how the height of a mountain can be calculated from 

 barometric readings. 



10. How many meters high is a mountain, if the barometric readings 

 at the same time at top and bottom are 564 mm. and 754 mm. respec- 

 tively? (A fall of 1 mm. corresponds to an ascent of 12 m.) 



11. If the mercury in a barometer stands at 76 cm. at sea level, 

 what would be the length of a column at an elevation of 3 miles? 



