In examples 4-27 the number given is either difference in time or difference in longitude between two places. In every case find the one not given.

28. One place is in 68° W.L. and another in 53° 15′ W.L. What is their difference in time?

29. Two places are in 120° 47′ and 13° 50′ east longitude respectively. What is their difference in time?

30. One place is in 83° 5' west longitude and another in 7° 16' 15'' east longitude. What is their difference in time? 31. It is 12 o'clock, midnight, at a certain place.

a. What is the time at a place 12° 15′ farther east? b. What is the time at a place 47° 18′ farther west?

32. When it is 2 P.M. at Paris, 2° 20′ 15′′ E.L.,

a. What is the time at Melbourne, 144° 57' 45" E.L.?

b. What is the time at Albany, 73° 44′ 45′′ W.L.?

33. What is the time at Cincinnati, 84° 26' W.L., when it is 11.50 A.M. at St. Louis, 90° 15′ 15′′ W.L.?

34. If I sail from Philadelphia, 75° 9′ 45'' W.L., with my watch set at the exact local time, and, after sailing a certain distance, find that my watch is 1 hr. 28 min. 40 sec. slower than the exact local time at that place, assuming that my watch has kept perfect time, what longitude has the ship reached?

35. A horse trotted a mile in 2 min. 15 sec.

a. During that time, the race track, on which the horse was traveling, moved how many minutes and seconds in its rotation about the earth's axis?

b. Estimating a degree of longitude at that place to be equal to 50 miles, how many miles did the race track move while the horse was trotting a mile?

36. a. A railroad train moving at the rate of 24 miles an hour, including stops, travels how far in a day?

b. The track on which the train runs moves how many miles a day, assuming a degree of longitude at that latitude to be 50 miles?

STANDARD TIME

636. The railroad companies of this country and Canada have agreed upon a division of the country into four time belts, extending north and south. All places in each belt take the time of the meridian which passes through or near the middle of the belt. This time is called standard time. The belts are as follows: Eastern, Central, Mountain, and Pacific.

A similar system of standard time is used in other parts of the world.

The standard meridian for the Eastern belt is the 75th, for the Central belt the 90th, for the Mountain belt the 105th, and for the Pacific belt the 120th.

These standard meridians are 15 degrees apart: when it is noon in the Eastern belt, it is 11 A.M. in the Central belt, 10 A.M. in the Mountain belt, and 9 A.M. in the Pacific belt.

In going westward from one time belt into another, the traveler sets his watch back one hour. In traveling eastward he sets his watch ahead one hour.

When it is noon on the standard meridian of a time belt, it is called noon at all places in the belt.

Standard time is not the true solar or local time, except for places situated on the standard meridians. Yet it can vary but little more than thirty minutes from the true time, and its uniformity is a convenience.

Standard time is used not only by the railroads, but also by people generally. The exact time is telegraphed daily to all sections of the country from the Naval Observatory at Washington.

637. Oral

1. When it is 5 P.M. Mountain time, what is the time in the Pacific belt?

2. When it is 11 A.M. Pacific time, what is the Central time? 3. In traveling from San Francisco to New York, how many times do I change my watch, and do I set it ahead or back?

4. When it is 4 A.M. at Augusta, Me., what is the standard time at St. Louis?

5. When it is 1 P.M. Mountain time at Denver, what time is it at Washington, D.C.?

6. What is the Pacific time at San Francisco when it is 5 P.M. at Chicago?

638. Written

1. What is the local time at Quebec, 71° 12' 15" W.L., when the standard time at that place is 7.30 A.M.?

2. What is the difference between local time and standard time in Chicago, whose longitude is 87° 36′ 42′′ W.?

3. When it is 6 P.M., standard time, at San Francisco, 122° 25′ 41′′ W., what is the local time?

639. Oral

REVIEW AND PRACTICE

1. Express in words: 4009; 350.01259; CXLVIII;

MCMX.

2. For what is the decimal point used?

3. Moving a figure three places to the left has what effect on its value? Two places to the right?

4. Moving the decimal point two places to the right has what effect on the value of the number in which it is placed ? One place to the left?

5. State three principles of Roman notation.

6. Describe two methods of testing results in subtraction.

7. Which term in subtraction corresponds to the sum in addition? It is the sum of what?

8. Which terms in multiplication are factors?

9. What is the shortest way to multiply an integer by 100? To multiply an integer by 7000?

10. 3675 x 100 = ? 600 × 7000 = ? 98 × 100 = ?

5. From 900,003.2 take 100.01.

6. Multiply 374 by 268 and read the partial products.

7. 468,316 is the product of 68, 71, and what other factor?

8. Find the value of 4837 + 32 x 1800 - 1728 72.

9. Find the value of (4837 +32) × (1800 — 1728) ÷ 72.

10. Find the prime factors of 36,465.

641. Oral

1. How many acres of land can be bought for $18,200, if every two acres cost $182?