WATER 23 



of water unite and form drops of water which fall to the earth in 

 the form of rain. If these drops are frozen, hail results. If the 

 vapor condenses in a region where the temperature is below the 

 freezing point, the moisture forms crystals of ice of various forms, 

 and these fall as snow. Moisture in the air serves another purpose 

 in helping to keep the earth warm. The lower and denser air is 

 heated by the sun's rays passing through them, and the moisture 

 acts as a blanket to prevent the loss of this heat by radiation. 



EXERCISES 



1. Place a glass tumbler of ice water in a warm room and note the 

 results. Repeat the experiment and direct a current of air against the 

 glass by fanning or by some other means, and note whether the amount of 

 moisture condensed on the surface of the glass is the same as before. 



2. Take two thermometers and see that they read the same in the begin- 

 ning. Now cover the bulb of one of the thermometers with a wet cloth 

 and place it in the wind where evaporation will be favored, and note whether 

 this thermometer reads the same as the one with the dry bulb. Account 

 for the difference. 



3. Weigh a potato, then heat it until all the water is driven off, and 

 weigh again. Compare the relative weights, and estimate the percentage 

 of solid mineral material and the percentage of water found in the potato. 



4. Weigh a green plant, then hang it up in the air for several days. 

 When it is thoroughly dry, weigh again. Note the difference and explain. 



QUESTIONS 



1. What is the composition of water ? 



2. What per cent of the plant is composed of water ? 



3. Discuss the amount of water required by a plant in making its growth. 



4. Do seasons affect this ? Explain. 



5. Discuss the distribution of rainfall in the United States. 



6. Discuss (a) evaporation, (6) dew, (c) fog, (d) frost. 



7. How many pounds of water are required to grow fifty bushels of 

 (a) oats, (6) wheat? 



8. How do clouds prevent frost ? 



9. What effect have winds on the formation of frost and dew ? 



10. Explain why plants on high ground frequently escape injury when 

 plants on low ground are frost-bitten and killed. 



REFERENCES 



Agriculture, Storer. 

 Elements of Physics, Hoadley. 

 Physical Geography, Dryer. 

 The Soil, King. 



