(5)

If you are in south latitude and sail north, Lat. left must be decreased by Diff. Lat.

(6) If you are in south latitude and sail north, and the Diff. Lat. exceeds the Lat. left, take the less from the greater, and the remainder will be the Lat. in, north.

The Lat. in, when it has been obtained by dead reckoning, is indifferently called the Latitude by Account, or Latitude by Dead Reckoning, and written in short Lat. by Acc., or Lat. by D. R.

III. To find the Middle Latitude

The Middle Latitude is the parallel of latitude midway between two places, hence it is half the sum of the two latitudes when they have the same name (N. or S.); half the difference when they have different names -one N. and the other S:

IV. To find the Difference of Longitude between two Places

RULE. If the longitudes of the given places be both East or both West, subtract the less from the greater; but if one be East and the other West, add them together, and the sum or remainder will be the difference of longitude. When the sum of the two longitudes exceeds 180 degrees, subtract it from 360 degrees, and the remainder will be the difference of longitude.

V. With the Longitude left, and Difference of Longitude, to find the

Longitude in

RULE. If the longitude left and difference of longitude are of different names, subtract the less from the greater, and the remainder will be the longitude in, of the same name with the greater; but if the longitude left and difference of longitude are of the same name, their sum will be the longitude in, of the same name with the longitude left. If, however, this sum exceed 180°, subtract it from 360°, and the remainder will be the longitude in, of a contrary name to the longitude left.

LONG. LEFT is the longitude from which the ship has departed.
DIFF. LONG. is the change of longitude in any interval.
LONG. IN is the longitude at which the ship arrives.

The method of obtaining the Long. in, from the Long. left and Diff. Long., is shown in the following examples and explanations:

Understanding that Longitude is reckoned E. or W. from the meridian of Greenwich to 180° :—

(1) If you are in east long. and sail east, Long. left must be increased by Diff. Long.

(2) If you are in east long. and sail west, Long. left must be decreased. by Diff. Long.

(3) If you are in east long. and sail west, and Diff. Long. exceeds Long. left, take the less from the greater, and the remainder will be the Long. in, west.

(4) If you are in west long. and sail west, Long. left must be increased by Diff. Long.

(5) If you are in west long. and sail east, Long. left must be decreased by Diff. Long.

(6) If you are in west long. and sail east, and Diff. Long. exceeds Long. left, take the less from the greater, and the remainder will be Long. in, east.

and 8) If the sum of the Long. left and Diff. Long. exceeds 180°, take the sum from 360° for the Long. in, which will have a different name from the Long. left.

Examples for Practice

I. Required the difference of latitude and longitude between Ushant in lat. 48° 27′ N., long. 5° 7′ W., and Cape Ortegal in lat. 43° 45′ N., long 7° 56' W.

Ans. Diff. of latitude 282 miles; diff. of longitude 168.

2. What is the difference of latitude and longitude between the Cape of Good Hope in lat. 34° 21′ S., long. 18° 30′ E., and St. Helena in lat. 15° 55′ S., long. 5° 44′ W.?

Ans. Diff. of latitude 1106 miles; diff. of longitude 1454'.

3. Required the difference of latitude and longitude between Cape Verde (paps) in lat. 14° 43′ N., long. 17° 34′ W., and Cape St. Roque in lat. 5° 29′ S., long. 35° 15′ W.

Ans. Diff. of latitude 1212 miles; diff. of longitude 1061'.

4. Required the difference of latitude and longitude between Cape Clear (Ireland) in lat. 51° 26′ N., long. 9° 29′ W., and St. Agnes Light (Scilly Islands) in lat. 49° 53′ N., long. 6° 20′ W.

Ans. Diff. of latitude 92 miles; diff. of longitude 1881.

5. A ship from Funchal, in Madeira, in lat. 32° 38′ N., long. 16° 55′ W., sails in the S.E. quarter until her difference of latitude is 326 miles, and difference of longitude 425′; required her present latitude and longitude. Ans. Latitude 27° 12′ N.; longitude 9° 50′ W.

6. A ship from latitude 2° 56′ S., and longitude 5° 14′ E., sails northwesterly until her difference of latitude is 352 miles, and difference of longitude 628'; required her present latitude and longitude.

Ans. Latitude 2° 56′ N.; longitude 5° 14′ W.

7. A ship from the Equator, and longitude 89° 17′ E., sails southwesterly until her difference of latitude is 370 miles, and difference of longitude 118'; required her present latitude and longitude.

Ans. Latitude 6° 10' S.; longitude 87° 19′ E.

8. A ship from Cape East (New Zealand) in lat. 37° 40′ S., iong. 178° 36′ E., sails in the N.E. quarter until her difference of latitude is 114 miles, and difference of longitude 297′; required her present latitude and longitude.

Ans. Latitude 35° 46′ S.; longitude 176° 27′ W.

9. Required the difference of latitude and longitude between Valparaiso in lat. 33° 2' S., long. 71° 41′ W., and Gutzlaff Island in lat. 30° 47′ N., long. 122° 11' E.

Ans. Diff. lat. 3829 miles; diff. long. 9968'.

MARINE SURVEYING

About one hundred and ten years ago John William Norie brought out the first edition of his "Epitome of Navigation." In the chapter on Marine Surveying he said

"The art of surveying coasts and harbours being very essential to those who visit unknown parts is treated in a manner which it is hoped will make its acquisition and practice perfectly easy. Notwithstanding the great importance of accurate surveys of various coasts and harbours frequented by mariners, it must be confessed that this branch of the nautical art has been little attended to, and that the opportunity which so frequently occurs to seamen is almost entirely neglected. We therefore think it proper to lay down a few general directions showing how a coast or harbour may be easily surveyed with such instruments as are used

at sea.

At the present time there are few places that have not been surveyed, but there are many places that have not been thoroughly surveyed, many places that were surveyed so long ago that alterations, sometimes of serious extent, have taken place, or the navigator may come upon an uncharted bank or rock, or obtain other information he desires to record. He should, therefore, know how to fix a position accurately upon a plan or chart, or if necessary construct a chart for himself and put in anything he desires. So, repeating Norie's words

We therefore think it proper to lay down a few general directions how to make a survey of a harbour, fix the position of a rock, shoal, or wreck.

INSTRUMENTS

CHRONOMETER whose rate on G.M.T. is known.

SEXTANT properly adjusted, and whose centering error is known.
ARTIFICIAL HORIZON for use on shore.

A HACK WATCH.

AZIMUTH and PORTABLE COMPASSES.

ANEROID BAROMETER marked in feet (mountain use).

CAMERA for Photographic observations.

A RELIABLE TAPE MEASURE and other measures,

FIELD'S PARALLEL RULERS.

STATION POINTER.

PROTRACTOR.

DRAWING BOARD and T SQUARE.

A number of pointed staves to mark off distances when measuring the base line.

Several long staffs with bunting at the end to mark special points.
A bucket or two of whitewash to help to distinguish a station.
Morse and Semaphore Flags for communication from a distance.

For Work on the Water

Patent Log.

Lead Line marked in fathoms and feet.

83

G 2

A painted pole marked in feet and half-feet for shallow water and, if there is time, for tidal observations.

A tidal gauge or pole with alternate feet painted black and white also showing half-feet or tenths of a foot.

Book to record angles, distances, depths of water, height of hills, and any other notes for plotting on the chart.

TO SURVEY A BAY OR HARBOUR.

Take a general view of the place either by walking or sailing round it in a boat or steam launch or by both.

During the journey make notes and rough sketches, or, better still, take photographs of the most prominent objects, headlands, bays, hills, or mountains, particularly when they come in line, and of anything conspicuous or remarkable.

Select a place suitable for a base line. A long level stretch of sand at sea level is the best, but it must not be land-locked, as it is necessary that as many objects, headlands, and points should be seen from both ends of the line as possible.

It is possible that suitable marks will have to be made by the use of staffs or otherwise. All the information should be marked on a rough sketch and the objects observed named, lettered, or numbered, in order to distinguish them when wanted.

The Base Line.

The fixing of the base line must be done with all possible accuracy, for an error in the base line will throw out all the observations referred to it or derived from it. The position should be selected with care either on the land, or when it is not possible to get a land base, then on the water. If not all, then most of the stations should be visible from each end of the line. Its length and direction should be such that the angle contained between it and any of the stations taken from one end of the base line may differ at least ten degrees from the same object taken from the other end thereof. Set up the two station marks as far apart as the ground permits, and accurately measure the distance between them by

Direct measurement,

Velocity of sound,

Astronomical observation,

Masthead angles,

Patent Log while under steam; or by a combination of more than one method.

Direct Measurement.

Having marked the observation station and called it A, select position of B, then stick into the ground at intervals flagstaffs sighted in exact line between them so that the measurers may get the true direction.

Measure with the tape 100 feet of well-used and stretched deep-sea leadline or sounding wire. Splice a ring one or two inches in diameter into both ends; let that length be 100 feet over all. Have a quantity of wooden