The geographer locates Cairo, Egypt, by saying it is on a meridian 31 degrees east of the prime meridian, and 30 degrees north of the equator, on a line parallel to the equator called a parallel of latitude. Find Cairo, in Figs. 10 and D. Geographers use the meridian that passes through the observatory at Green wich (a part of London) as the prime meridian, and measure distance east and west of that in degrees of longitude, and distances north and and noon travel westward many miles a min ute? How many miles a minute? Since we have the habit of meeting trains we need to agree on how we shall set our clocks. For convenience we divide the United States into bands or belts (Fig. E) about 15 de grees wide. In the eastern belt every body keeps 75th meridian time. In the central belt everybody keeps 90th meridian time. The lines between the belts are ir regular because it would be very incon venient for the conductor of a train to change his time a few miles outside of a city like Columbus, Ohio, or Salt Lake City, Utah. Therefore we zigzag the edges for the sake of convenience. Every time a traveler going west crosses into a new time belt he sets his watch backward one hour. If he goes east ward he sets it forward one hour.
15. International date line.—Suppose he went westward clear around the world, setting his watch back an hour every fifteen degrees. When he got back home, he would be one day behind the calendars of the home folks. Those twenty-four hours that he had lost by moving his watch back would have taken a day, and he would therefore say it was Saturday when it was really Sunday. To avoid this trouble men have made an international date line out in the Pacific Ocean (Fig. 10) where it will make the least trouble. Every time you cross that line going west you move your calendar back one day. Going east you move it ahead one day.
16. Change of seasons.—To understand our change of seasons, do the following: On a sheet of paper mark a place near the cen ter, and call it sun. Around this draw a line to show the path of the earth. Now stick a pin into the paper somewhere in your path of the earth. This pin represents;the earth's axis. The flat sheet of paper represents the plane of the earth's path or orbit. The axis of the earth points to the same place in the sky all the time, and it is not perpendicular to the plane of the orbit. Look at Fig. F and see that the axis is slanting. Remember that the earth goes clear around the sun, and you can see that the North Pole is in darkness when it is inclined away from the sun during northern winter. On December 21st, our shortest day, the sun shines on the surface of the earth no farther north than the place marked by the Arctic Circle. At that time a place just south of the Arctic Circle has only a few minutes of sunshine each day, and places inside theArctic Circle have many days with no sun at all.
When the earth is on the other side of the sun, June 21st, the North Pole (Fig. F) is inclined toward the sun, and tEe sun shines clear across it to the Arctic Circle on the other side. At this time a place just south of the Arctic Circle has only a few minutes daily when the sun cannot be seen, and places north of the Arctic Circle have many days when the sun does not set at all. This makes the Arctic tundra have such surprising plant life (Sec. 355). The Antarc
tic Circle is 231 degrees from the South Pole.
The Tropic of Cancer, 231 degrees from the equator, is the most northerly place on which the sun's rays fall perpendicularly in June. The Tropic of Capricorn is the same distance south of the equator. Can you tell why? 17. Equinox.—On the 21st of March and the 21st of September the sun shines on both poles (Fig. F). The days and nights are every where of equal length, and for that reason we say it is equinox.
18. The zones.—Geographers have divided the earth into belts called zones (Fig. D) be cause of the differences in sunshine at different seasons. The zone along the equator between the tropics of Cancer and Capricorn is called the torrid zone because of its heat. Around each pole there is a frigid zone, 'so named because of its cold. The Arctic Circle bounds the northern, and the Antarctic Circle the southern frigid zone. Between each frigid and the torrid zone there is a temperate zone, so called because its average temperature is between that of the torrid and that of the frigid zones. Find all the zones on Figs. F and G. In which zone do you live? In which zone is it winter when we have summer.
19. The wind systems of the world.— Examine this drawing (Fig. G) carefully, and you will see that at the equator, where the air is very hot and therefore light, it is ascending. (See edges of drawing.) At high elevations it turns and travels back toward the poles, while surface winds, the trade winds, come from the northeast and the southeast to take its place (Fig. 540).
Just north of the Tropic of Cancer and again south of the Tropic of Capricorn is a region of descending air where there is so little wind that sailing vessels sometimes have trouble to keep sailing along. Sailors call these places the Horse Latitude calms.
North of the Northern Horse Latitudes and south of the Southern Horse Latitudes the wind blows so nearly from the west that these zones are called the prevailing wester lies. These westerlies blow very regularly, but it is hard for the people who live in the midst of these winds to understand that they are in the zone of the westerlies because of the great eddies or cyclones, several hundreds of miles across, that come along every few days. One of these cyclones is shown in the westerly wind zone of the Northern Hemisphere (Fig. G). It blows along with the westerly winds, but around its center the wind is in all directions. For a full account of the cyclones see Secs. 59 to 72, and of the trade winds, Secs. 741, 744.
20. Ocean the wind blows over water it rubs against the 'surface. This rubbing or friction draws the water along with the wind. Thus the ocean has currents. Look at Fig. 327. What direction have the currents in the Atlantic and Pacific oceans where the prevailing westerlies blow? where the trade winds blow? The currents in the Indian Ocean north of the equator blow one way in winter and another in summer. Examine Figs. 509 and 510 and see if you can not explain why these currents thus change.
How do the North Atlantic and North Pacific currents help you understand why the climate is so much alike on the east sides of the two oceans and also on their west sides?