purpose of adopting a universal first meridian, the one passing through the Royal Observatory at Greenwich, England, was selected.

11. The latitude of any place on the earth's surface is the distance north or south from the equator measured on the meridian that passes through the place. Thus, if a place is situated at A, Fig. 3, north of the equator EE', its latitude is the arc, or

distance, BA of the meridian PB P' that passes through the place. It is named north or south latitude according as the place is situated to the north or south of the equator; thus the latitude of A is north because A is situated in the northern hemisphere. Again, if a place is situated at C in the southern hemisphere, its latitude

is the arc DC of the meridian PD P' and it is named south. Latitude is reckoned from the equator toward the poles in degrees, minutes, and seconds, and since the distance from the equator to either pole is 90°, the latitude of any place can never exceed that amount. When a ship is on the equator, its latitude is 0°; if at the north pole, its latitude would be 90° N; if at the south pole, 90° S.

12. Parallels of latitude, or latitude parallels, are small circles whose planes are parallel to the equator. Every point on the circumference of a latitude parallel is equidistant from the equator, consequently all places situated on the same latitude parallels have the same latitude. Like meridians, latitude parallels can be drawn through any place on the earth.

13. The difference of latitude of any two places is the arc of a meridian contained between the two latitude parallels passing through those places. In Fig. 3, the difference of latitude between Fand G is the arc FG of the meridian contained between the latitude parallels passing through the two places.

14. All places situated on those latitude parallels have the same difference of latitude. Thus, the difference of latitude between A and F is the same as that between Fand G. For example, the difference of latitude between A and C, Fig. 3, is the arc G C or AḤ.

The difference of latitude between B and F, Fig. 3, is the arc FD or JB.

15. To Find the Difference of Latitude.-If the places are both on the same side of the equator, or, in other words, their latitudes have the same name, the difference of latitude is found by subtracting the smaller latitude from the greater; when the two places are situated on opposite sides of the equator, that is, when their latitudes have different names, the difference of latitude is obtained by adding the two latitudes.

EXAMPLE 1. - Find the difference of latitude between Boston, Mass., in latitude 42° 22′ N and Philadelphia, Pa., in latitude 39° 56′ N.

SOLUTION. - The latitude of both having the same name their difference is taken; thus,

EXAMPLE 2.-The latitude of Cape Verde is 14° 43′ N, that of Cape St. Roque is 5° 28' S. Find the difference of latitude between the two places.

SOLUTION. -The latitudes being of different names their sum is taken; thus,

EXAMPLE 3. - The latitude of Southampton, England, is 50° 54′ N; the latitude of New York is 40° 42.7' N. Required the difference of latitude between the two places.

16. When a ship sails north in northern latitudes and south in southern latitudes she increases her latitude; but, when sailing south in northern latitudes and north in southern latitudes she decreases her latitude. Therefore, when the difference of latitude and one latitude is given the other is readily found by addition or subtraction.

NOTE. By latitude left is understood the latitude of the place the ship sailed from; by latitude in, the latitude of the place arrived at.

EXAMPLE 1.-A ship sails from a place in latitude 27° 15′ S, a distance of 320 true north. Required the latitude of place arrived at.

SOLUTION. - The ship having sailed north in southern latitudes, her latitude in is evidently less than her latitude left.

EXAMPLE 2.-A ship from the west end of the island of Maderia, in latitude 32° 48′ N, sails a distance of 98' true north. Find her latitude in.

SOLUTION. - The ship sailing to the north in northern latitudes her latitude in is evidently greater than the latitude left.

EXAMPLE 3.-A steamer from Michigan City, in latitude 41° 43′ N, runs 137' true north.

SOLUTION.

What is the latitude in?

EXAMPLES FOR PRACTICE

1. A ship from latitude 26° 17' S sails 190 south. latitude in.

=

Find her Ans. Lat. in 29° 27 S. 2. The latitude of one place is 40° 40′ N, of another 33° 42′ N. Find the difference of latitude. Ans. Diff. of Lat. = - 6° 58', or 418'. 3. The latitude left is 3° 2' S, the difference of latitude sailed is 190' north. Find the latitude in. Ans. Lat. in = 0° 8' N. 4. The latitude left is 2° 48′ S; the difference of latitude sailed is 288' north. Find the latitude in. Ans. Lat. in 5. The latitude left is 3° 42′ S; the latitude in is 1° 40′ N. difference of latitude sailed. Ans. Diff. of Lat. 6. The latitude left is 0° 10′ N; the difference of latitude 228' north. Find the latitude in. Ans. Lat. in 7. A ship is in latitude 68° 48′ N and another in 38° 30′ N. is the difference of latitude between the two?

=

= 2° 0' N.
Find the

= 322' N.

sailed is 3° 58' N. What

Ans. Diff. of Lat. = 30° 18', or 1,818'.

17. The colatitude is the complement of the latitude, or 90° latitude. Thus, in Fig. 3, the colatitude of the place A is the arc PA; if the latitude of A is 75° N, its colatitude is 90° — 75°:

=

15°.

18. A nautical mile is reckoned as 6,080 feet, or 1,013 fathoms; it is equal to the mean length of a minute of latitude. A statute mile is less than a nautical mile and contains only 5,280 feet.

19. The longitude of any place is the distance in arc east or west, measured on the equator from the first meridian to the meridian passing through the place.

Thus, in Fig. 4 (a), if PB P' represents the first (Greenwich) meridian, then the arc BD of the equator EE' intercepted between the first meridian and the meridian passing through G is the longitude of that place, and is named west because it lies to west of the first meridian. If G had been to the east of PB P' its longitude would have been so many degrees east.

Longitude may also be defined as the angle at the pole subtended by the first meridian and the meridian passing through the place. This angle GPF, the arc GF of the

parallel, and the arc BD of the equator, we know, contain the same number of degrees, minutes, and seconds, although the linear

distance of BD is greater than GF.

20. Longitude is reckoned from 0° to 180° east or west, but is never considered greater than 180° either way; if it exceeds 180°, it is subtracted from 360° and given the contrary name. Thus, longitude 186° W is equal to 360° — 186°

E

21.

Conversions of Time and Angular Measure.

Since longitude can be expressed either in time or in angular measure, it is necessary to be able to reduce time to angular