referve fuch Objects for the next measured Bafe; for when Lines lie very oblique to one another, their Interfections are not easily afcertained.

Thus may a Coaft of any Extent be furveyed, by carefully measuring of Stationary Bafe Lines, and from their Ends drawing Angles and Lines to cut each other..

If any particular Parts of the Harbour cannot be conveniently feen from either of the Stations, take the Boat into thofe Places, and having well examined them, make Sketches thereof, eftimating the Length and Breadth of the feveral Inlets, either by the rowing or failing of the Boat, taking as many Bearings, Soundings, and other Notes, as may be thought neceffary; then annex these particular Views in their proper Places in the general Draught.

If there are any dangerous Sands or Rocks, befides inferting them in their proper Places, there fhould be a double Line drawn through that Point, or one or more Objects afhore; and for this Purpose chufe a Church, Mill, Houfe, noted Tree. a Clift, or any remark, able Thing that can be diftinctly feen at Sea, and which can be brought to bear in the fame right Line with the Point to be avoided; but if that Point is under Water, there must be two Land Marks brought to bear with the Danger, either in a right Line, when it can be, or in two Lines; and that thofe two Lines, and that thofe Land Marks may be put down in their proper Places, their Bearing muft alfo be taken from two of the Ship's Stations.

It should be remarked in the Draught what Places, if any, are unfit for Anchorage, and what are fit, by writing " rocky Ground, foul Anchorage, good Anchorage ;" and in the latter, to draw the Figure of an Anchor. Alfo if there is any particular Channel more convenient to fail through than another, it is to be pointed out by Lines drawn to its Entrance, from two or more noted Marks athore.

The foregoing Method of furveying a Coaft fuppofes in general, that it is taken by a Ship in her Paflage along, not having an Opportunity of going afhore. But when Circumftances will permit the Measures and Obfervations to be made on Land, the Survey can be taken more accurately than on the Water.

To furvey an Harbour by Obfervation on Shore,

Make an Eye-Draught of the Place to be furveyed; and in going round its Coaft fix in the moft remarl:able Points and Bends of the Shore, Station Staves, or ftrait Poles, tall enough to be seen at a confiderable Distance: But if at any of thefe places there is a noted Tree, Houfe, or any other remarkable Thing, that Object may serve instead of a Station Staff; and it will be convenient to black the Staves, and tie a Piece of white Bunting to the Top of each: Then in the Eye-Draught put Letters at the noted Points, or Marks, for Diftinction fake.

Chuse the most level Spot of Ground, wherein a bafe Line may be measured, of one or more Half Miles in Length, or a Length of not less than a tenth Part of the Distance of the two extreme Objects marked for obferving, and let the Direction of the measured Bafe Line be fo laid out, that from both Ends of it as many as poffible of the Station Staves before planted, or the Objects before remaxked, may be feen. The Bearing or Pofition of this Bafe must be well determined by Degrees and Minutes; and alfo its Length must be accurately measured to Feet and Parts, either by a measured Chain, or by a Piece of Log-line of 100 Feet long, properly marked at the End of every 10 Feet, and each End Length marked at every Foot.

From one End of the Bafe obferve (with any Inftrument proper to take Bearings) the Pofition or Bearing in Degrees and Minutes, of all the Staves or Objects within View, and write them down orderly; do the fame from the other End of the Base, and let all the Bearings be corrected by the Variation of the Compafs.

Then, thefe Measures and corrected Bearings being plotted or laid down, will give the most confpicuous Points of the Shore; the intermediate Spaces are to be filled up from the Sketches of them made on the Spot,

But if any of these Objects should spread on either Hand fo far beyond the Limits of the Bafe, that at either End thereof, the other End and thofe Objects or Staves fhould appear nearly in the fame Direction, or to make Angles not exceeding about 10 Degrees: Or, if fome of the remarked Objects can be feen only from one End of the Bafe, then let the Bearings of such Objects be taken from a Place whofe Pofition has been determined from both Ends of the measured Bafe; or if there are feveral remarked Objects, which cannot be feen from neither End of the Bafe Lines, let the Bearings of fuch Objects be taken from each of the two Points, whofe Pofition has been' taken from both Ends of the Bafe. Or, it may on fome Occafions, be proper to chufe another Place, on which another Bafe of a convenient Length may be meafured, and from the Extremities of which the Ends of the firit Bafe may be feen; and alfo as many as can be of the remaining Objects which lay too obliquely for the first Bafe, or which could not be feen from it. In fuch Manner proceed, until the Bearings are taken of all the Points judged neceflary for completing the Survey of the Limits of the Harbour.

If a Bafe of a fufficient Length cannot be measured in one right Line, it may be taken in two adjoining Lines, as the two Sides of a Triangle; the included Angle being accurately taken, and the Bearing of either Line.

When the Outlines or Limits of an Harbour, Bay, Road, &c. are delineated by the preceding Precepts; Let a small Veffel go out to Sea, and take Drawings of the Appearance of the Land and its Bearings, Sail like wife into the Harbour, and draw the Appearance

of

of its Entrance; take particular Notice if there are any falfe Refemblance of the Entrance, by which Ships may be deceived and run into Danger; take the Bearings, or when any two Objects being brought in a line or in one, will lead into the Harbour without Danger, when it can be done, fearch for the beft anchoring Places, and if poffible denote thofe places by bringing two Objects in one, if not, the exact Bearings of two or three other Objects, so that the Places may be easily determined, the Chart being correctly drawn ; a Compaís, with the Variation and Scale properly fitted to the Plan; the Ifles, Rocks, Sands, &c. marked in their proper Places; the Settling and Drift of the Currents and Tides; the Times of High Water on the Days of Full and New Moon, with the Rife of Water at thofe Times, and whether whole Tide, and Tide and Part; the beft anchoring Places with their Soundings at low Water, and the Winds open to them; the beft Track, with the Soundings all the Way to thofe anchoring Places; the proper failing Marks to avoid Dangers: the Winds, if any troublefome ones, which prevail, and at what Seafons; the Places where fresh Water can be got; the Name of the Place, the Country in; on what Sea; the Latitude and Longitude; a Sketch of the Appearance the Place makes at Sea upon a known Rhumb, and at an estimated Distance; and whatever elle a judicious Seaman fhall think proper to infert; then is the Plan fit for all nautical Purposes, and may be embellished with proper Colours, if necessary.

Sea Draughts taken according to the foregoing Precepts, and neatly drawn and coloured, befides the real use they may be of, cannot fail to recommend the young Mariner, who furveys and constructs them, to the Notice of his Superiors.

To reduce a Draught to a larger or fmaller Scale.

With a Black Lead Pencil draw the Draught to be reduced all over with cross Lines, forming exact Squares: Draw the clean Paper for the Copy alfo over with the fame Number of Squares, but their Sides larger or fmaller in Proportion to the intended Size of the Scale; fuch as one-half, one-fourth, &c. of the Length of the other. Diftinguish by a stronger Line and Mark with a Figure every fifth or fixth Row of Squares in both, fo that the feveral correfponding Squares may be readily perceived; then in each of the Squares of the Draught, draw, by the Eye, a Curve on the Paper fimilar to that in the Square in the Draught, till the whole is copied, make the black Lines with India or other Ink, and when drawn, the Black-Lead Lines may be rubbed out with Bread, or India Rubber,

To find the Height and Diftances of Objects at Sea.

When the Object is perpendicular, and the Distance to it can be measured, find the Angle of Altitude with a Quadrant, and measure the Distance to it as exact as poffible, then you have the Angles and

Bafe, to find the Perpendicular; or, if you go backward or forward until the Angle of Altitude be 45°, the Distance between you and the Object will be the perpendicular Height.

EXAMPLE.

Being 96 Fathoms from the Bottom of a Tower, I find its Altitude (after allowing for the Height of my Eye above the Water) 15°10'; required the Height?

Draw AB=96, upon B erect the Perpendicular BC, and draw A C, making an Angle

with AB 15° 10′ till it cuts BC

15-10

B

96

in C; then will BC be the Height of the Tower 26.2.

When the Object is not perpendicular, fuch as Hills, Mountains, Rocks, &c. the perpendicular Height may be found by observing the Altitude at any convenient Distance, and then going either farther off, or nearer to it, in a straight Line, and then obferving the Altitude again. With these two Angles, and the Distance between the two Stations, being carefully measured, the perpendicular Height may be found either by Geometry or Trigonometry.

EXAMPLE.

Being at Sea, I obferved the Altitude of a Mountain, and found it 8°; and then failing towards it in a direct Line 3 Miles, I found the Altitude of the fame Mountain to be 12° 30': Required the perpendicular Height?

8 by drawing AC. Set off from A to B 3 Miles, or 5280 Yards, and at make the Angle DBC 12° 30', by drawing the Line BC to cut the Line AC in C. From C let fall the Perpendicular CD, which being measured, will give the perpendicular

Height of the Mountain 2027 Yards. To find which by Calcula

tion:

Given the Angles DAC-8°, and DBC= 12° 30'; then 12° 30' fubtracted from 180° leaves Angle AEC 167° 30', which being added to Angle A 8°, the Sum 175° 30' fubtracted from 180°, leaves the Angle ACB 4° 30', whence we have the Angles; and one Side AB 5280 given, to find the Side AC; then it will be

In like Manner may the Height or Distance of any acceffible or inacceffible Object be found, either at Sea or on Shore; and the Angles may be taken with a Quadrant, Semi-circle, &c.

NOTE. The Reafon why Angle B is fubtracted from 180° is, because that when a Right Line meets with another Right Line and makes Angles, the two Angles will be equal to 180o. (Eucl. l. 13.)

Of the Curvature of the Earth.

Most Persons know that if they are raised above the Surface of the adjacent Land or Water, they can not only fee different Objects that lie on that Surface better, but also fee thofe more and more remote as they advance higher. The Irregularity of the Surface of the Land will not be fubjected to any one Rule that will give the Diftance to which Objects may be feen at different Elevations; but at Sea, where there is generally an uniform Curvature of the Water, upon the Suppofition of the fpherical Figure of the Earth, thofe Diftances may be eafily computed.)

RULE. To the Earth's Diameter add the Height of the Eye, multiply the Sum by that Height; then the Square Root of the Product is the Distance at which an Object on the Surface of the Water can be feen by an Eye fo elevated; and by this Rule was the Table annexed computed, the Diameter of the Earth being taken at 41798117 Feet, according to Sir Ifaac Newton's Meafures. This Table may be usefully applied to eftimate the Distance of an Object at Sea, the Elevation of that Object above its Horizon being known.

Sailing towards a Head-land, on which is a Light-Houfe, elevated 600 Feet above the Surface of the Water, we few the Lights at Night juft appearing in the Horizon. How far were we at that Time diftant from the Light-Houfe?