152. To find the latitude in, having given the latitude from and true difference of latitude.

RULE XLVI.

1. When the latitude from and true difference of latitude have a like name-To the latitude from add the true difference of latitude (turned into degrees, minutes, and seconds, if necessary): the sum will be the latitude in, of the same name as the latitude from.

2o. When the latitude from and true difference of latitude have unlike names-Under the latitude from, put the true difference of latitude (in degrees and minutes, if necessary); the remainder marked with the name of the greater is the latitude in.

EXAMPLES FOR PRACTICE.

Find the latitude in, in each of the following examples :

1. Lat. from 31°10' N.

3.

3 2 S.

• 39 N.

190 N.

59 S.

8.

357 N.

9.

"" 4 48 S.
35 25 S.

153. To find the middle latitude, having given the latitude from and latitude in. (For definition see No. 132, page 86.)

RULE XLVII.

The name being supposed alike, that is, both North or both South-Add together the true latitudes, and take half the sum; the result is the middle latitude. NOTE.-When the names are unlike, the middle latitude (which is seldom required but for obtaining the departure) should be found by means of a table; but in this case, it may perhaps be as well to avoid the use of the middle latitude in any of the common problems of navigation.

Required the middle latitude in each of the following examples:

154. To find the difference of longitude, having given the longitude from and longitude to. (For definition see No. 134, page 86).

RULE XLVIII.

1o. When the longitudes are of the same name— -Take their difference and reduce the same to minutes, place E. or W. against the remainder, according as the longitude to is East or West of longitude from.

2o. When the longitudes are of contrary names-Take the sum of the two longs., which sum, if less than 180°, is the diff. of long., and attach E. or W., according as the long. to is East or West of long. from; but when the sum exceeds 180° subtract it from 360°, for the diff. of long., and reduce the remainder thus found to minutes, attaching to it the contrary name to that found in the usual way.

Longitudes are reckoned East or West of the first meridian. If these different directions are considered one positive and the other negative, the difference of longitude of two places is always found by taking the algebraic difference of their longitudes.

Required the difference of longitude between a place A and a place B in each of the following examples:

155. To find the longitude in, having given the longitude from and the difference of longitude.

RULE XLIX.

1o. When the longitude from and the difference of longitude have like names-To the longitude from add difference of longitude (turned into degrees, if necessary); the sum, if not more than 180°, will be the longitude in, of the same name as the longitude from; but if the sum exceed 180°, subtract it from 360°, and the remainder is the long. in and of a contrary name to long. from.

2o. When the longitude left and difference of longitude have unlike names -Under longitude from, put difference of longitude (in degrees and minutes, if necessary); take the less from the greater; the remainder, marked with the name of the greater, is the longitude in.

EXAMPLES FOR PRACTICE.

Required the longitude in, or arrived at, in each of the following examples:

13. Define meridian of the earth, equator, parallel of latitude.

156.

The Compass is simply an instrument which utilises the directive power of the magnet. A magnetised bar of steel, apart from disturbing forces and free to move, points in a definite direction, and to this direction all others may be referred, and a ship guided on any desired course.

There are various adaptations of the instrument, according to the use it is specially intended for. The compass intended for use on board ship is called the "Mariner's Compass," and according to the purposes it is intended for it is named The Steering Compass, The Standard Compass, and The Azimuth Compass.

157. The Mariner's Compass consists of a circular card, which represents the horizon of the observer; the circumference or edge of the card being divided according to two systems of notation into points and degrees.

The origin of the compass is very obscure. The ancients were aware that the loadstone attracted iron, but were ignorant of its directing property. The instrument came into use in Europe sometime in the course of the thirteenth century.