NAVIGATION.

TH

HE great End and Bufinefs of Navigation is to inftruct the Mariner how to conduct a Ship through the wide and pathlefs Ocean, to the remotest Parts of the World, the fafeft and shortest Way, in Paffages navigable.

For the due and regular Performance of which are requifitea perfect Knowledge of the Figure and Motion of the Earth, the various real, and imaginary Lines upon it, fo as to be able to ascertain the real Distance and Situation of Places with refpect to one another, with the Ufe of the feveral Inftruments made Ufe of in meafuring the Ship's Way; fuch as the Log, Half-minute Glass, Quadrant, to take the Altitude of the Sun and Stars; Compafs, to reprefent the fenfible Horizon; and Azimuth Compass, to take the Azimuth or Amplitude of the Sun, in order to know the Variation of the magnetic Needle; Maps or Charts of the Seas and Lands together with the Depth of Water, and the Times and Settings of the Tides upon the Coafts he may have Occafion to approach near; a competent Knowledge of Currents; of the Mould and Trim of the Ship, and the Sail fhe bears, that fo due Allowance may be made for Leeway: By Help of these, and Skill in the Navigator, he may know at all Times the Place the Ship is in, which Way he must fteer, and how far, to gain his intended Port.

Notwithstanding what has been faid, it may not be improper here to obferve, that,

As Latitude is counted from the Equator upon an Arch of the Meridian, North and South, the Difference of Latitude between two Places, both North or both South, is found by fubtracting the lefs Latitude from the greater; but if one Latitude be North and the other South, the Difference is found by adding both Latitudes together.

Confequently, if a Ship in North Latitude fails Northerly, or in South Latitude Southerly, fhe encreases her Latitude; but in North Latitude failing Southerly, or in South Latitude failing Northerly, fhe decreases her Latitude; because the fails nearer to the Equator, from whence the Latitude is reckoned.

Wherefore in North Latitude failing Northerly, or in South Latitude failing Southerly, the Difference of Latitude added to the Latitude left, gives the Latitude in.

In North Latitude failing Southerly, or in South Latitude failing Northerly, the Difference of Latitude fubtracted from the Latitude left, gives the Latitude in.

When the Latitude decreases, and the Difference of Latitude is greater than the Latitude failed from, fubtract the Latitude left from the Difference, and the Remainder will be the Latitude in, and of a different Name; for it is plain, in this Cafe, that the Ship has crof fed the Equator.

As the Longitude is counted from the first Meridian Eaft and Weft, until it comes to the oppofite Meridian, therefore it cannot exceed 180 Degrees.

The Difference of Longitude between two Places, being both Eaft or Weft, is found by fubtracting the lefs Longitude from the greater; but if one be in Eaft Longitude and the other in West, their Sum is the Difference of Longitude.

Therefore in Eaft Longitude failing Eafterly, or in Weft Longitude failing Wefterly, the Difference of Longitude added to the Longitude left, gives the Longitude in.

In Eaft Longitude failing Wefterly, or in Weft Longitude failing Eafterly, the Difference of Longitude fubtracted from the Longitude left, gives the Longitude in.

When a Ship fails Eaft or Weft, until fhe paffes the oppofite Meridian, or 180 Degrees, the changes her Longitude, or comes into `a Longitude of a different Name.

What has been faid, will be rendered familiar to the Learner by the following Examples:

In the last Example it is plain, that as the Difference of Latitude is more than the Latitude left, the Ship muft have crossed the Equa tor, and confequently come into South Latitude.

Note. When one of the Places has no Latitude, or is on the Equator, then the Lat, of the other Place is their Difference of Latitude.

EXAMP. V. What is the Difference | EXAMP. VI. A Ship from Cape of Longitude between Cape Finifterre and East Point of Barbadoes?

Charles in Virginia fails Weltward, till her Difference of Lon gitude be 400 Miles, what Longitude is the in?

59: 37W.

50:23W.

60

In failing due North or South, the Ship changes her Latitude only; and failing Eaft or Weft her Longitude; but failing upon any other Course, the muft change both Latitude and Longitude.

Eafting or Wefting in Plane Sailing is called Departure or Meridian Distance.

The Inftrument ufed in measuring a Ship's Way at Sea, is the Log.

Ships at Sea are directed from one Place to another by Means of an Inftrument called the Mariner's Compafs.

The Mariner's Compass is an Artificial Representation of the Horizon of every Place, by the Means of a circular Piece of Paper, called a Card, divided like the Horizon into Degrees and Points, which are called Rhumbs. Now the Card being properly fixed to a Piece of Steel, called the Needle, that has been touched with a Loadstone (whofe Property is fuch as to cause one End of the Needle fo touched to point towards the North, when turning freely on fomething supporting it) all the Points of the Card will be directed towards the correfponding Points of the Horizon:

NOTE. All the Cafes in Plane failing may be stated thus:

MSM

MS

NE

5

NEGYE

ENE

E6YN

7

East

Elys

-8

ESE

SEGYE

SE

Sways

SSW

Sbyw
South

$ by E

SSE

SElys

2

3

A

2

Hence it follows, that in every Place the North Point of the Card fhews the Pofition of the Meridian at that Place, and fome one Rhumb or Point of the Card will coincide with, or be directed along the Track that makes any given Angle with the Meridian; confequently, by the Help of the Card or Compafs, a Ship may be kept in any propofed Track or Course.

A Rhumb Line, or Point, is a Right Line drawn from the Centre of the Compafs to the Horizon, and is named from that Point of the Horizon it falls in with,

The Course is the Angle which any Rhumb Line makes with the Meridian, and is fometimes reckoned in Degrees, and fometimes in Points of the Compafs; fo that if a Ship fails upon the second Rhumb or N. N, E. the Courfe is 22 Degrees 30 Minutes: And fo for any other, as in the following Table, which the Learner fhould be fo well acquainted with, or the Compass, as to be able readily to tell how many Points any Courfe or Rhumb is diftant from the Meridian, or from the Parallel,