xii CONTENTS 



CHAPTER XIII. 

 The Weather 232 



Causes of Weather. Changes in Density and Pressure of Air. 

 Heating of the Air. Moisture of the Air. Humidity. Dew and 

 Frost. Fogs and Clouds. Forms of Clouds. Rain and Snow. 

 Hail. Rainfall. Exercises. The Winds. Regular Win ds. 

 Storms and Cyclones. Thunderstorms. Tornadoes. Cyclones of 

 the Tropics. Weather Service. Weather Maps. Summary. 

 Exercises. 



CHAPTER XIV. 



Rocks and Soil 258 



The Earth's Crust. Some Common Rocks. Classes of Rocks. 

 Origin of Stratified Rocks. Origin of Igneous and Metamorphic 

 Rocks. Weathering of Rocks. Causes of Weathering. Drift. 

 Erosion. Exercises. Soil. Structure of Soil. Tilling the Soil. 

 Irrigation. Fertility. Loss of Fertility. Preserving and Re- 

 storing Fertility. Rotation of Crops. Artificial Fertilizers.-* 

 Summary. Exercises. 



CHAPTER XV. 

 Plants 282 



Plants and the Soil Functions of Plants. Germination of a 

 Bean. Other Seeds. Leaves. Work of Leaves. Modified 

 Leaves. Stems. Structure of Stems; Wood. Sap. Buds. 

 Roots. Underground Storage of Food. Flowers. Formation of 

 Seeds. Dispersal of Seeds. Exercises. Classes of Plants. 

 Algae. Fungi. Mosses. Ferns. Seed Plants. Economic 

 Plants. Distribution of Plants. Summary. Exercises. 



CHAPTER XVI. 

 Animals 312 



What is an Animal? One-Celled Animals. Simple Many-Celled 

 Animals. Starfishes. Worms. Mollusks. Crustaceans. In- 

 sects. Exercises. Fishes. Amphibians. Reptiles. Birds. 

 Mammals. Importance of Animals to Man. Summary. Exer- 

 cises. 



