116 PHYSICAL GEOGRAPHY. gradual crystallization while the moisture is condensing. Sleet is frozen rain. The snow line is the distance above the sea where snow remains throughout the year. The snow line in the tropics is found at about three miles above the level of the sea; in the temperate regions, at rather less than two miles; near the northern extremi- ties of the continents, at less than one mile; while still farther north, on the polar islands, it is but a few hundred feet above the sea. ‘ The height of the snow line, depends— (1.) On the amount of the snowfall. (2.) On the temperature of the valley. (3.) On the inclination of the slopes. Glaciers are immense masses of ice, formed by the snow ‘which accumulates on the slopes of mountains above the snow line. They move slowly by gravity.down the moun- tain slopes, bearing with them accumulations of dirt and stones, called moraines. The upper surface of the glacier is generally broken into deep fissures, called crevasses. The water derived from the melting of the glacier issues in a stream from the lower end of the ice mass. It is highly charged with sediment derived from the erosion of the glacier. It often forms the source of a powerful river. The following mountains contain glaciers: the Alps, the Pyrenees, the Caucasus, the Scandinavian Mountains, the Himalayas, and the Karakorum. When glaciers descend into the sea, the waters under- mine them, and detach huge masses, which float away to great distances. These masses are called icebergs. Toward the close of the Mammalian Age, a change oc- curred in the climate of the earth, by which all the north- ern contineuts were covered with glaciers. The unit of electric potential is called a volt; the unit of current is called an ampére; the unit of resistance is called an ohm. Comparing the flow of electricity to that of a current of water in a pipe, the volt corresponds to the pressure ‘causing the flow, the ohm to the friction or other resist- ance opposing it, and the ampére to the quantity of the flow per second. The free electricity of the air is generally positive. Lightning results when the electricity of a cloud dis- charges to the earth, or to a neighboring cloud. There are five kinds of lightning: zig-zag, heat, sheet, globular, and volcanic. When the air contains an unusually great quantity of electricity, faintly luminous balls are seen on the ends of tall objects. These are called St. Elmo’s fire. Auroras are caused by the passage of electricity through the rare air of the upper regions of the atmosphere. The earth acts like a huge magnet. It possesses a mag- netic field, and has lines of force entering its south pole in the Northern Hemisphere, and coming out of its north pole in the Southern Hemisphere. A magnetic needle, if free to move, will come to rest in the earth’s field with the lines of force of the earth pass- ing in at its south pole and coming out at its north pole. The magnetic needle points to the north, from the action of the magnetic poles of the earth. The cause of the earth’s magnetism is not certainly known. It is probably due to electrical currents which circulate around it. Magnetic storms, or unusual variations in the earth’s magnetism, correspond with outbursts of solar activity as manifested by sun-spots. The deviation of the needle from the true north, is called its declination; the deviation from a horizontal plane, its inclination. Both declination and inclination are subject to diurnal, annual, and secular variations. Isogonal lines connect places which have the same dec- lination. Isoclinal lines connect places which have the same inclination. Isoclinal lines are nearly coincident with the isothermal lines. Rainbows are caused by the action of light on falling raindrops. Halos are caused by snow crystals in the air; Coronez, by minute particles of water. The Mirage is caused by the bending cf the rays of light from their original direction, while passing from one me- dium to another of different density. REVIEW QUESTIONS. ——-005,00— What do you understand by evaporation? Name the circumstances upon which the rapidity of evaporation depends. Define dew point. What condition is necessary in order that the invisible moisture of the atmosphere may become visible in any form of precipitation ? Under what circumstances is dew deposited ? Why is more dew deposited on a clear night than on a cloudy night? Why is more dew deposited on a still night than on a windy one? ; Under what circumstances are fogs, or mists, produced? How do fogs or mists differ from clouds? What is the condition of:the particles of water which form the clouds? Are they minute drops, or hollow vesi- eles? Describe the appearance of the cirrus cloud. How does its height compare with that of other clouds? During what parts of the day are stratus clouds most common? To what do they owe their banded appearance? Describe the cumulus cloud. During what part of the day is it most common? Why should the cirro-stratus clouds generally indicate approaching rain? Name three conditions under which rain may be caused. By which are the heaviest rains generally produced ? Ave the equatorial currents likely to bring rain or drought? The polar currents? Why? Name the periodical rain zones, When does it rain in the zone of calms? In the zone of the trade winds? Why? In what portions of the zone of the variable winds is the rainfall approximately periodical? Describe the rainfall in the zone of the variable winds. In the zone of the polar winds. Describe the construction of a rain-gauge or pluvi- ometer. Why should more rain fall on a mountain than on the lowlands at its base? Why should more rain fall on the _ coasts of a continent than in the interior?