Due to the rotation of the earth and the division of the equator into 360 degrees, the sun apparently passes over 24 of 360° or 15o each hour. Similarly it appears to pass over 1° every 4 minutes. Rule. - To find the difference in time in minutes, mul . tiply by 4 the number of degrees difference in longitude. Rule. — To find the difference in longitude in degrees, . divide by 4 the number of minutes difference in time. EXAMPLE 1. Find the difference in time between Boston and San Francisco if the difference in longitude is 51 degrees. 51, difference in longitude 4 204 minutes, or 3 hr. 24 min., difference in time EXAMPLE 2. Find the difference in longitude between London and New York if there is a difference in time of about 5 hours. 5 hours is 300 minutes 75, the number of degrees difference in longitude 4 (300 minutes difference in time Exercise 1. Through how many degrees does the earth turn in 1 hour ? in 5 hours ? 2. When it is noon at Washington, what time is it at a place 15° 30' east of Washington ? 45° 45' west? 3. What is the difference in longitude between two places whose difference in time is 1 hour, 10 minutes ? 4. If a person travels until his watch is 2 hours fast, in what direction and through how many degrees has he traveled ? 5. What is the difference in time between two places whose longitudes are 75° east and 60° east? 75° east and 60° west ? 75° west and 60° west ? Antwerp LONGITUDES OF VARIOUS CITIES Naples 82° 22' W. Sydney 14° 14' E. 74° 00' W. 79° 32' W. 2° 20' E. 80° 03' W. 12° 29' E. 122° 25' W. 90° 12' W. 151° 12' E. 139° 44' E. 71° 39' W. 77° 03' W. . . . Exercise 1. When it is noon at San Francisco what time is it at Antwerp? at Berlin ? at Boston ? at Buenos Aires ? 2. When it is noon at Constantinople what time is it at Capetown? at Chicago ? at Denver? at Havana ? 3. When it is 6 A.M. at New York what time is it at Honolulu? at Manila ? at Mexico City ? at Naples ? 4. When it is 3 P.M. at Panama what time is it at Paris ? at Pittsburgh ? at Rome? at San Francisco? 5. When it is midnight at Washington what time is it at St. Louis ? at Sydney ? at Tokyo ? at Valparaiso ? Standard Time For practical purposes a great part of the civilized world is now divided into sections, all places in the same section using the time of a certain meridian in that section. To make the times of adjacent sections just one hour apart, the meridian usually chosen is 15°, 30°, 45°, or other multiple of 15°, east or west of the meridian through Greenwich. Thus, England uses the time of Greenwich, that is, 0°; Central Europe uses that of 15° E.; Japan uses that of 135o E.; and the Pacific Slope of the United States that of 120° W. A time system where all places in a section of territory near some chosen meridian have the same time is called a system of standard time. The standard time map of the United States is shown on the following page. The irregular boundaries of the sections are due to a desire to have the division lines pass through important railway centers. When it is noon by Eastern time it is 11 A.M. by Central time. When it is noon by Eastern time it is 10 A.M. by Mountain time. When it is noon by Eastern time it is 9 A.M. by Pacific time. Exercise 1. When it is noon in California what time is it in Vermont? in Kentucky? in Wyoming ? in eastern Arizona ? 2. When it is 6 P.M. in Massachusetts what time is it in Michigan ? in Oklahoma ? in Utah ? in Florida ? 3. When it is midnight in Iowa what time is it in Montana ? in Washington ? in Ohio ? in New York ? 4. When it is 6 A.M. in Maine what time is it in Alabama ? in Minnesota ? in Oregon ? in Virginia ? 5. When it is 9:30 A.M. in Colorado what time is it in Florida ? in Pennsylvania ? in California ? in Texas ? 6. An order from San Francisco to Boston is wired at 2 P.M. The wire takes i hr. to reach Boston, where the office of the recipient closes at 6. Will the telegram be received before closing time? Board and Lumber Measure The board foot is the unit used in measuring lumber, and is the amount of lumber in a piece one foot wide, one inch thick, and one foot long. Rule. To find the number of board feet in a piece of lumber, multiply 1 of the number of square inches in an end section of the piece by the number of feet in the length. EXAMPLE 1. Find the number of board feet in a piece of lumber 1 foot wide, 2 inches thick, and 18 feet long. 2 x 12 x 18 = 36, the number of board feet in the piece. 12 |