REVIEW AND MAP QUESTIONS. 117 Compare the mean annual rainfall of the tropics of the Old and New Worlds. Of the temperate regions of the Old and New Worlds. Name the rainless districts of the Eastern Continent. Of the Western Continent. What is the cause of the almost total absence of rain in these districts? Under what circumstances is hail produced? Describe the structure of a hailstone. Explain the rotary theory of hail. Define the snow line. Upon what does the height of the snow line depend? At what height above the sea- Jevel is it found in the tropics? In the temperate re- gions? In the polar zones? How are glaciers formed? In what respects do they resemble rivers? What are crevasses? How are they formed? Name some rivers which take their origin in the melt- ing of glacial ice. Define lateral moraines; medial moraines; terminal moraines. Explain the manner in which fiord-valleys were formed. What is the probable origin of lakes in all glacier districts? Name some of the European mountain systems which contain extended glacier regions. Name two Asiatic mountain ranges which contain such regions. How are icebergs formed? Is the ice of which they are composed salt or fresh ? What are ice floes? State their origin. What appears to have been the southern limit of the glaciers in the United States, during the glacial epoch, which occurred toward the close of the Mammalian Age? What is the origin of free atmospheric electricity ? Define volt; ohm; ampére; potential; circuit. Under what circumstances does lightning occur? What is the cause of the accompanying thunder? Name five varieties of lightning. By what are auroras caused ? What is the cause of the directive tendency of the mag- netic needle? What is believed to be the cause of the earth’s magnet- ism? What do you understand by the earth’s magnetic field? Define isogonal lines; isoclinal lines. With what lines are the isoclinal lines nearly coinci- dent? Explain the phenomenon of the rainbow. What is the cause of the sunset tints of the sky? Of the blue color of the sky? What are halos and corone? By what are they caused? Explain the cause of the mirage of the desert. What do you understand by the phenomena of loom- ing? MAP QUESTIONS. ——+030300— Trace on the map of the winds and rains, the portions of the world included in the zone of calms. When does it rain in the zone of calms? Trace in a similar manner the portions included in the zones of the trades, and the zones of the variables. What is characteristic of the rainfall in each of these zones ? Why should the eastern shores of tropical South Amer- ica be moist, and the western dry? To what peculiarity of position does Northern Africa owe its scanty rainfall? Trace on the map of the isothermal lines the southern limit of the Arctic drift ice; the northern limit of the Antarctic drift ice. Trace on the declination chart, the agone, or line of no declination, in the Western Hemisphere. Trace the line of no declination in the Eastern Hemi- sphere. What smaller line of no declination exists in this hemisphere ? Notice that in the Western Hemisphere the isogonal lines all meet in a point near Hudson Bay. What does this meeting indicate?