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LONGITUDE AND TIME.

224. Every circle is supposed to be divided into 360 equal parts, called degrees.

Since the sun appears to pass from east to west round the earth, or through 360°, once in every 24 hours, it will pass through 4 of 360°, or 15° of the distance, in 1 hour; and 1° of distance in of 1 hour, or 4 minutes; and 1' of distance in of 4 minutes, or 4 seconds, etc.

TABLE OF LONGITUDE AND TIME.

360° of longitude = 24 hours, or 1 day of time.

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225. To find the difference of time between two places, when their longitudes are given.

1. The longitude of Boston is 71° 3', and of Chicago 87° 30'. What is the difference of time between these

two places?

OPERATION.

87° 30'

71°

3'

16° 27' 4

=

SOLUTION. By subtraction of compound numbers we first find the difference of longitude between the two places, which is 16° 27'. Since 1° of longitude makes a difference of 4 minutes of time, and 1' of longitude a difference of 4 seconds of time, we multiply 16° 27′, the difference in longitude, by 4, and we obtain the difference of time in minutes and seconds, which, reduced to higher denominations, gives 1 h. 5 min. 48 sec., the difference in time. Or, since places distant from each other 15° of longitude differ 1 h. in time; 15' 1 minute in time, etc., of 16° 27' is the difference in hours, minutes, and seconds of time.

65' 48" 1 h. 5 min. 48 sec. Or, 15) 16° 27'

1 5 48

h.

min. sec.

PRAC. AR. -14

RULE.-Multiply the difference of longitude in degrees and minutes by 4, and the product will be the difference of time in minutes and seconds, which may be reduced to hours. Or,

Divide the difference of longitude in degrees, minutes, and seconds by 15, and the quotient will be the difference of time in hours, minutes, and seconds.

1. Multiplying by 4 gives an answer 60 times as great as dividing by 15, but since by the first operation the answer is in minutes (which we then reduce to hours), and by the second in hours, the first number should be 60 times as great as the second, hence we see that both the above methods give the same result.

2. If one place is in east, and the other in west longitude, the difference of longitude is found by adding them, and if the sum is greater than 180°, it must be subtracted from 360°.

2. New York is 74° 1', and Cincinnati 84° 24' west longitude. What is the difference of time?

Ans. 41 min. 32 sec. 3. The Cape of Good Hope is 18° 28' east, and the Sandwich Islands 155° west longitude. What is the difference of time? Ans. 11 h. 33 min. 52 sec.

4. If a message is sent by telegraph without any loss of time, at 12 м. from London, 0° 0′ longitude, to Washington, 77° 1' west, what is the time of its receipt at Washington?

Since the sun appears to move from east to west, when it is exactly 12 o'clock at one place, it will be past 12 o'clock at all places east, and before 12 at all places west. Hence, knowing the difference of time between two places and the exact time at one of them, the exact time at the other will be found by adding their difference to the given time if it is east, and by subtracting if it is west.

Ans. 6 h. 51 min. 56 sec. A.M.

5. Washington is 77° 1' west, and St. Petersburg 30° 19' east longitude. What is their difference of time? Ans. 7 h. 9 min. 20 sec.

6. A steamer arrives at Halifax, 63° 36' west, at 4 'clock, P.M.; the fact is telegraphed to St. Louis, 90° 15' west, without loss of time.. What is the time of its receipt at St. Louis ?

Ans. 2 h. 13 min. 24 sec. P.M.

7. If Pekin is 118° east, and San Francisco 122° west longitude, what is their difference of time?

8. If, at a presidential election, the voting begins at sunrise and ends at sunset, how much sooner will the polls open and close at Eastport, Me., 67° west, than at Portland, Oregon, 124° west? Ans. 3 h. 48 min

9. When it was 1 o'clock, A.M., on the first day of January, 1891, at Bangor, Me., 68° 47' west, what was the time at the city of Mexico, 99° 5' west?

Ans. Dec. 31, 1890, 58 min. 48 sec. past 10, P.M. 226. To find the difference of longitude between two places, when the difference of time is known.

1. If the difference of time between New York and Cincinnati is 41 min. 32 sec., what is the difference of longitude?

OPERATION.

min. вес.

4) 41

32

10° 23', Ans.

min.

41

Or,

sec.

32

15

623' 0"

=

10° 23', Ans.

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SOLUTION. Since 4 minutes of time make a difference of 1° of longitude, and 4 seconds of time, a difference of 1' of longitude, there will be as many degrees of longitude as there are minutes of time, and as many minutes of longitude as there are seconds of time. Or, since there are 15 times as many degrees, minutes, and seconds of longitude as there are hours, minutes, and seconds of time, we multiply the time by 15.

RULE. Reduce the difference of time to minutes and seconds, and then divide by 4; the quotient will be the dif ference in longitude, in degrees and minutes. Or,

Multiply the difference in minutes and seconds of time by 15, the product will be the difference in longitude in minutes and seconds.

Dividing by 4 gives an answer as great as multiplying by 15, but since by the Arst operation the answer is in degrees, and by the second in minutes (which we then reduce to degrees), the first number should be as great as the second; hence we see that both methods are correct and give the same result.

2. What is the difference of longitude between the Cape of Good Hope and the Sandwich Islands, if the difference of time is 11 h. 33 min. 52 sec.?

Ans. 173° 28'. 3. What is the difference of longitude between Washington and St. Petersburg, if their difference of time is 7 h. 9 min. 20 sec.? Ans. 107° 20'.

STANDARD TIME.

227. To avoid the inconvenience arising from the difference in time between places comparatively near (as the difference of 12 minutes between New York and Boston), the following standard time divisions have been adopted by the railroads, and local time is now generally superseded by standard time.

228. Standard Time Divisions as adopted by the Railroads. EASTERN STANDARD. 75th meridian. Canada, between Quebec and Detroit; U.S. east of Buffalo, N.Y.; Pittsburgh, Pa.; Wheeling and Huntington, W.Va.; Bristol, Tenn.; Charlotte, N.C., and Augusta, Ga.

CENTRAL STANDARD. 90th meridian. West from "Eastern" limits, as above, to Broadview, Canada; to the Missouri River in Dakota; North Platte and McCook, Neb.; Wallace and Dodge City, Kan.; Toyah and Sanderson, Tex.

MOUNTAIN STANDARD.

105th meridian. West

from "Central" limits to Heron, Mont.; Ogden,

Utah; Needles and Yuma, Ariz.

PACIFIC STANDARD. 120th from "Mountain" limits to coast.

meridian. West

229. As a matter of fact, standard time coincides with local time in very few places in the United States, and therefore for accurate calculations the correction is constantly needed.

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Eastern standard time - that of the 75th meridian-coincides very nearly with local time at Potsdam and Herkimer, N. Y.; Camden and Cape May, N.J., and Philadelphia, Pa.

that of the 90th meridian

Central standard timeis practically local time for Bessemer, Mich.; Mt. Carrol, Ill.; St. Louis, Mo.; Memphis, Tenn.; Jackson, Miss.; and New Orleans, La.

Mountain time is very nearly local time at Cheyenne, Wy.; Denver, Col.; and Las Vegas, N.M.

Pacific time is very close to local time at Reno and Carson City, Nev., and Santa Barbara, Cal.

While the standard meridians are exactly 15° apart, the belts using the several meridian times vary, according to convenience, from 10° to 25° in width. This results in some singular variations. For instance, El Paso, Tex., has the same standard time as Savannah, Ga., though there is a difference of 1 h. 45 min. in local time. On the other hand, San Diego, Cal., has two hours later standard time than El Paso, although there is a difference of but 42 min. in local time.

By knowing the meridian, which is the standard for any place, and the distance of the meridian from the place in degrees east o west, local time can be converted into standard time.

For a place east of its standard meridian, the time of the rising or setting of any heavenly body may be expressed in standard time by subtracting from the almanac calendar time one minute of time for every 15' of longitude, or 4 minutes of time for every degree of longitude that the place is east from the standard meridian.

For a place west of its standard meridian, the time of the rising or setting of the sun or moon may be found by adding to the almanac calendar time one minute of time for every 15' of longitude, or 4 minutes of time for every degree of longitude that the place is west of the standard meridian.

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