Longitude is distance east or west of the prime meridian. When we say that the longitude of Paris is 2° 20' East, we mean that the meridian running through Paris is 2° 20' east of the prime meridian that runs through Greenwich, England. The longitude of Washington, D. C., is 77° 7' West. That means that the meridian which passes through Washington is 77° 7' west of the prime meridian. For the Pupil to Do: 1. Find the longitude of your home town. 2. Find the longitude of Chicago. 3. Find the longitude of Denver. 4. Find the longitude of Salt Lake City. 5. Find the longitude of San Francisco. The longitude of a place tells in degrees, minutes, and secunds, the distance it is east or west of the prime meridian. Philadelphia is 75° 9′ W. longitude and Portland, Me., is 70° 15' W. longitude. The difference in longitude is 4° 54'. We subtract to get the difference in longitude. Paris is 2° 20' E. longitude and Philadelphia is 75° 9' W. longitude. The difference in longitude is 77° 29'. Here we must add to get the difference in longitude. If the pupil does not understand why we add in the first case, and subtract in the second, explain by marking off inches on a globe. Ask him how many inches two places are apart, when both are on the same side of the prime meridian. When on opposite sides. Take the map of the United States and show the pupil the sections of country in Eastern Time, Central Time, Mountain Time, and Pacific Time. The pupil should know the number of degrees between the meridian lines that bound these Time Belts. New York people are in the Eastern Time Belt. If they rise at 6 o'clock in the morning, they will be up a whole hour before Chicago people, who get up at the same hour. Thus, each day begins an hour sooner in New York than in Chicago, two hours before Denver, and three hours before San Francisco. Ask the pupil to tell what time belt he lives in. If he has any friends or relatives in any of the other time belts of U. S., have him tell what time they have at the present moment. WHERE THE DAY BEGINS. The day begins earlier as you go east from New York, until you meet the 180th meridian. This is where the day begins. Starting here, it travels westward, giving the whole world a new day. The 180th meridian is called the International Date Line (I. D. L.) but in reality, the date line is a crooked line which zigzags across the 180th meridian. From the time the day starts at the I. D. L., until the sun again reaches that line, the same day is in progress the world over. Location of the International Date Line. The Pathfinder in 1899 showed a diagram of the I. D. L., as its course was then marked. Since then it has been changed. As marked now, it passes southward through Behring Sea, then westerly, then returns to the 180th meridian at about 40° north. It then follows the 180th meridian to 10° south, where it swerves east but returns again to the 180th meridian at about 50° south. It then follows that meridian. The date line is not 30 crooked as before the change. The sun appears to move in a circle around the earth, passing through 360° in 24 hours: Then, In 24 hr., the sun passes through 360° of 360° = 15° In 1 min., the sun passes through of 15° = 15′ Every 15° that one passes through, going either east or west, makes a difference of one hour in his time. To reduce longitude in time to longitude in degrees, multiply by 15; to reduce longitude in degrees to longitude in time, divide by 15. TALK: PROBLEMS. Two kinds of problems are generally given in longitude and time, along with those in which one is asked to merely find differences in longitude and time. They' are (1) Given the longitude of two places and the tim at one of them, to find the time at the other; and (2) Given the times of two places and the longitude of one, to find the longitude of the other. All will be explained here. PROBLEM: Find the difference in time between Brooklyr 73° 58′ W. and Omaha 95° 28′ W. WORK: 95° 28' 15) 21° 30' difference in longitude. PROBLEM: What is the difference in longitude between twc places whose difference in time is 2 hr. 33 min 12 sec. ? It is noon Thursday, 130° W. (just west of San Francisco). What time is it 160° E. (in Siberia)? WORK: 130° 15) 290° 191 hr. 19 hr. 20 min., the difference in time. Day comes earlier 160° E. Therefore, it is 7.20 A. M. Friday, in Siberia. PROBLEM: When it is noon at Greenwich, what time is it at Philadelphia ! 15) 75° difference in longitude. 5 hr. difference in time. It is 5 hr. earlier in Philadelphia, or 7 A. M. For the Pupil to Do: 1. When it is 3 o'clock P. M. at Philadelphia, where is it 2 P. M.? where is it 1 P. M. where is it noon ? 2. When it is 10 A. M. at Greenwich, what time is it at Boston at Chicago? at San Francisco ? PARTICULAR MEASURES. NOTE. These are not in frequent use. Neither are the measures always definitely known. A hand, used in measuring the height of horses, is 4 inches. A stone of lead or iron is 14 pounds. A span is 9 inches. A square, for measuring floors, etc., is 100 square feet. A degree of longitude at the equator is 69.16 miles. A degree of longitude on the 40th parallel is about 53 miles. A fathom, used in measuring depth at sea, is 6 feet. A knot, or geographical mile, or nautical mile, is 1.15 common miles. A carat, used in weighing diamonds, weighs 3 gr. Troy. A. H.-17 |