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O B L I Q U E S A I L I N G.
As the Sine Angle B * 39° 23' 9.80228 Is to the Side A C s- 15 Miles I. 17609 Sois Sine Angle A o- 78° 45' 9.99.157 - 11. 16766
To the Side C B 23.19, to the Distance of the Ship 1.36538
from the Cape at her second Station - ape Q. CAS E.
In the Triangle D A C are given the Side A C 16.31, the Side
A D 14.35, and the Angle DAC 43 Points = 53° 26', the Angles'
* . — . . . . . . . . U then by Axiom Diff. of Sides 1,96 Half Sum of ditto 63 17's third it will be,
As the Sum of the Sides A C and A D : - -30,66 1,48657
Is to their Difference I 39 o,29226 So is T. of ; the Sum of Angles ADC and ACD 63°17. ... 10,26816 - - * ...' .- ... —– . - IO,59042 . - - - - 1,48657 - - - - - - - 4. --- * To the Tangent of half their Difference 7° 14' . 9,10385:
*** *t, i... . . . . . Q 2. . . . . . . . . . . For ... 3
To the Side D C 139 the Dist. between the Head-lands 1,14286
All the above Proportions may be worked by Gunter's Scale, but they are more readily done by Proječtion.
After this Manner may the mutual Bearings and Distances of any ?Number of Head-lands be found; as also the Maps of Harbours, *Sea Coasts, &c, which are of excellent Use in reconnoitering the Enemy's Coasts, Towns, &c. without venturing too near their Forts. Many more Examples might be produced, which are not of so much Use at Sea, and therefore are omitted.
There is a Method of ascertaining the Distances of Places by Sound, which is as follows: Fix a Pendulum of 38.2 Inches upon, a Peg or Nail, and giving it a Swing, every time it passes the Centre will give Seconds; and as it is found by many Experiments, that Sound travels 1142 Feet in one Second; therefore the Difference of Time between seeing the Flash and hearing the Sound being measured by the Pendulum, the Distance may be easily found by allowing 1142 Feet for every Second, or allowing 9.3 Half Seconds to. travel an English Mile, or five Seconds three Tenths to travel a Geographical Mile. If a Stop Watch is at Hand it will answer the Purpose better.
Being at Sea, I counted 50 Seconds between the Time of seeing the Flash of a Gun and hearing the Report. I demand the Distance 2.
If 50 Seconds be divided by 5,3, the Quotient 9,3 Miles, will be the Distance required; or if the same. Number of Seconds, viz. 50,. be divided by 16, the Quotient 3 one eighth Leagues, will give the Leagues of Distance, so that the Gun, when it was fired, was 9,4 Miles, or 3 Leagues; and one eighth from the Observer.
Hadley's Quadrant or a Sextant will be found-more convenient for taking the Angular Distances of Objećts in running along a Coast, when the Angles do not exceed 90° or 120°, as they can be more accurately and expeditiously taken, and the Draught of a Sea Coast or Harbour, as well as the Distances of Objects, be determined with sufficient Exačtness: For having a Base or Stationary. Line, accurately measured, and the Angles taken from different Parts of it, the Intersections of the Line, including these Angles, will determine
the several Points of the Coast or Harbour, and Sketches may be drawn to represent the Appearance of the Land, &c. If Time will permit, the Soundings, Rocks, &c., may be taken in a Boat, and the Bearings of the whole afterwards laid down } the Compass. Even such a rough Draught as the above will be of great Use in giving an Idea of Coasts and Harbours of any Country we chance to fall in with, and if the Mariner has a Taste for Drawing he may give a pretty good Description of the Parts he has been at, a Thing but too much neglected by our English Seamen.
Co are certain Settings of the Stream, by the Means W. A of which all Bodies moving therein, are compelled to alter their Course, and submit to the Motion impressed upon them by it. Whence, if a Current sets with the Course of the Ship, it augments her Motion by as much as the Drift or Rate of driving it. Thus, if a Ship sails N. N. E. 20 Miles in a Current that sets N. N.E.8 Miles in the same Time, her true Course will be N. N. E. 28 Miles in that Time; but if a Current sets against the Ship, it lessens her Velocity by just so much as the Current’s Drift is: So that if a Ship sails N. E. 49 Miles'; in a Current that sets S. W. 10 Miles in the same Time, then her true Course will be N. E. 39 Miles; and if, in the same Time that the Ship sails N. E. 49 Miles, in a Current that sets S. W. 59, then the Ship will fall a-stern, and her true Course will be S. W. io Miles; but if the Ship thwarts the Current, it not only lessens or augments her Velocity, but gives her a new Motion compounded of that of the Ship and Current; that is, If a Body be agitated by two Motions A at the same Time, the one with a certain *... Velocity, that will carry it according to: the Direction of the Line A B, the ‘... Length a Bin a certain Space of time; P —o' C. the other according to the Direétion of the Line A D, with a Velocity that will carry, it to the Distance A D in the same Time; then the Body will describe the Diagonal A C, and at the End of that" Time will be found in the Point C. The Setting and Drift of the most remarkable Tides and Currents are pretty well known ; but if in unknown Currents, the usual Way to find the Setting and Drift, is thus: Let three or four Men take a Boat a little Way from the Ship, and by a Rope fastened to the Boat's Stem, let down a heavy Iron Pot, Or