14. London, St. Paul's Cathedral, 0° 5′ 38′′ W., and New York, City Hall, 74° 0′ 24′′ W.

Solution: New York 74° 0′ 24′′ 73° 59' 84"

=

15. San Francisco 122° 25′ 42′′ W. and Boston 71° 3′ 50′′ W. 16. Newark, N. J., 74° 10' 0" W. and Manila 120° 58′ 6′′ E. 17. Pittsburgh 80° 2' 0" W. and Cape Town 18° 28′ 45′′ E.

110. Time. The earth may be regarded as a large sphere which turns around an imaginary line passing through the poles. It makes a complete revolution, or turns 360°, in 24 hours.

It is noon at a certain place when the meridian which passes through it is directly under the sun.

In one hour this place has passed over an arc equal to of 360°, or 15°. Why?

In one minute it has passed over of 15°, or 15'. In one second it has passed over an arc equal to 15′′. Using the fact that 360° of longitude correspond to a complete turn, prove the following corresponding values of time and longitude:

Longitude..

Time.

360° 15° 15' 15" 1°

1'

1"

24 hr. 1 hr. 1 min. 1 sec. 4 min. 4 sec. 15 sec.

EXERCISES

1. When it is 10 A.M. in Chicago, determine the corresponding local time of a place 22° east of Chicago; 18° west of Chicago; 25° east of Chicago.

Suggestion: To 1° of longitude correspond 4 minutes of time. Hence to 22° correspond (22×4) minutes.

2. Find the number of degrees of longitude corresponding to 8 seconds of time; 10 minutes; 30 minutes; 42 minutes.

3. Find the time corresponding to a change in longitude of 20°; 10'; 2° 15'.

111. Local time. Since the earth turns from west toward east, the objects in the sky, which may be

regarded as fixed in position, seem to pass over us from east toward west. Therefore if it is noon where we are, it is earlier, or before noon, at all places west of us, but it is later, or after noon, at all places east of us.

Thus for all places having the same longitude, i.e., lying on the same meridian, the time is the same; but it is different for places on different meridians. Time reckoned this way is called local time.

Years ago people used to check the accuracy of their watches by comparison with a sun dial. Some of the

口四

FIG. 216. NOTE THE TIME

BALL ON TOP OF THE TOWER

larger cities distributed time by dropping a time-ball (Fig. 216) situated on a high tower to indicate exact noon. Today it is not necessary for a person to compare his watch directly. Time is computed by astronomers and distributed all over the country by telegraphic transmission.

EXERCISES

1. Important events happening in France during the war were read in the St. Louis dailies at an earlier hour than was indicated as the time of sending the cable message. Explain how this happened.

2. The longitude of certain buildings in Chicago and New York are 87° 36′ 42′′ W. and 74° 0′ 3′′ W., respectively. When local time is noon at New York, what is local time at Chicago?

Solution:

Difference in longitude = 13° 36′ 39′′.

Using the table in §110, show that this is equal to

(13×4) min.+(36×4) sec.+

sec.

=52 min.+2 min. +24 sec.+2.6 sec.

= 54 min.+26.6 sec.

.. The local time at Chicago is 54 min., 26.6 sec. before noon.

The table below gives the longitude of some important buildings of several large cities:

Places

Washington San Francisco Boston

Denver

Longitude.. 77° 3′ 0′′ W. 122° 25′ 42′′ W.71° 3′ 50′′ W. 104° 58′ W.

3. On a steamer crossing the Atlantic the clocks are moved ahead 32 minutes a day. What is the change in longitude each day?

4. A man going abroad finds that after several days spent on the ocean his watch is 3 hours and 42 minutes slow. By how much has the ship's longitude changed?

5. When it is 8 A.M., local time, in Washington, find the time in Boston.

6. When it is noon in Denver, find the time in San Francisco. 7. When it is 3 P.M. in Denver, find the time in Washington.

8. The time in Denver is 2 hours 3 minutes 56 seconds earlier than in New York. Find the difference in longitude.

9. Two ships at sea are 68° 12′ 25′′ of longitude apart. What is the difference in time?

112. Standard time. When it was customary to use only local time, each locality had to determine its own time. This was satisfactory

as long as travel was slow and infrequent. The telegraph and railroads brought about changes, however, which made it necessary to regulate the time in a more satisfactory way than formerly. A person traveling east or west had to set his watch continually to have the local time of the cities

at which he was stopping. Because railroads were inconvenienced in the same way, the principal railroads of the United States adopted a uniform system of keeping time. They divided the country into four

time belts (Fig. 217). These belts are approximately 15° wide. The dividing lines are irregular and depend largely on the location of railroad terminals. With each belt the time changes one hour. The Eastern Belt

extends on both sides of the 75th meridian and all places within it have the same time as the local time of the 75th meridian. The Central Belt has the local time of the 90th meridian. The Mountain Belt and Pacific Belt take the time of the 105th and 120th meridian respectively.

EXERCISES

1. How much faster are the clocks in any time belt than the clocks in the next time belt to the west?