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Mariners reckon by the civil Account of Time ufed on Shore, but they keep the Reckoning for the Ship's Place, by beginning at Noon, and counting from thence 24 Hours to the next Noon. From Noon to Midnight they mark with P. M. fignifying after Mid-day; and the fecond 12 Hours with A. M. fignifying after Midnight; ending their Day's Work at the Noon of the civil Day. Hence their Ship's Account is 12 Hours earlier than their Shore Account of Time. And as the Sun's Declination ufed for finally determining the Ship's Place at the End of the Sea Day is calculated for the Noon of the common Day at London or Greenwich, therefore the Declination for the Noon of the civil Day, must be taken for determining the Latitude, &c. at finishing their Day's Account. Thus a Day's Work marked Tuesday, May the 6th, began on Monday at Noon, and ends on Tuesday Noon, so that the Sun's Declination for the 6th of May is used for the Noon of Tuesday, and fitted to the Meridian of the Ship, according as she is E. or W. of London.

There are various Methods of keeping a Sea Journal, according to the Sentiments of various Perfons with Regard to what deferves being recorded: Some approve of a Journal including the Log-book, each Day's Work at some Length, and fuch Occurrences as feem of moft Importance; while others prefer a fhort Abftract of this long Journal, containing little more than the Course run, the Latitude and Longitude in, and fometimes the Bearing and Distance of the intended Port for each Day.

In the following Journal the long Form is ufed as reprefenting more fully each Day's Work, and the neceffary Corrections; and an Abstract of this may be drawn out in the fhortest Form that feems confiftent with Diftinctness. The Learner ought to be thoroughly acquainted with the long Form, and when he does that, he may either continue it, or take the fhorteft Form; or retrenching from the firft, and adding to the fecond, what Particulars he thinks proper, and thereby make out a Form adapted to his own particular Tafte.

RULES

RULES for correcting the DEAD RECKONING by an Obfervation.

NOT

OTWITHSTANDING the Rules already laid down for keeping a Ship's Way at Sea, yet by Reafon of the feveral Accidents that may attend a Ship in one Day's Run, fuch as fwelling Seas, different Rates of failing between the Times of heaving the Log, want of Care at the Helm in letting the Ship yaw or fall off, accidental Currents, fudden Squalls, when no Account can be kept, &c. the Latitude by Account and Latitude by Obfervation may very often differ, then it is neceffary that proper Corrections be made in the Difference of Longitude.

When you have made all proper Allowances you can, fuch as for Leeway, Variation, Currents, &c. and still find that your Latitude by Account will not agree with your Latitude by Obfervation, then you must correct as follows:

Firft, Confider whether you have made proper Allowances for Currents, Heave of the Sea, if the Course at the Helm has been carefully attended to, if the Log-line and Half-minute Glass be juft, and the Log properly hove, or any fudden Squalls, or proper Allowances made for the Leeway, &c. which of thefe you conjecture your Error is in; make what Allowances you think meet to your Difference of Latitude and Departure by Dead Reckoning, and fee if that will reform your Latitude by Account, fo as to make it agree with your Latitude by Obfervation; if it does, you have gueffed right; (for you must always keep to the Latitude by Obfervation, it being the only Thing to be depended on ;) but if it will not agree with the observed Latitude, it is to be supposed that there are Miftakes in your Conjecture, or fome other Caufe which produces the Error in the Reckoning, and ftands in need of being corrected. In this Cafe, you are firft to examine your Log-line and Half-minute Glafs, and if there be an Error in them, allow for it, as in the following Example.

E XAM PIE I.

Yesterday, at Noon, we were in Latitude 48° 20' N. and till this Day at Noon we have failed S. S. W. 48 Miles, S. W. by S. 36 Miles, N. E. 24 Miles, and find by a good Obfervation that we are in Lat. 47° 14' N.

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By the Traverse Table it appears that by Account_the_Diff. of Lat. is 57.3 S. and the Departure 21.4 W.

Now the Lat. left was

Diff. of Lat. by Account

Latitude in by Account

48° 20' N.

。 57 S.

47 23 N.

Differing 9 Miles from the true Latitude by Obfervation. Wherefore I examine the Log-line and Half-minute Glafs, and find that the former measures 52 Feet between Knot and Knot, and that the latter runs only 27 Seconds. Now as the Log-line and Halfminute Glass are both faulty, I correct my Difference of Latitude and Departure as in Cafe III. and find my correct Difference of Latitude 66.2 S. and my Departure 24.7 W.

Now from Latitude left

Take Diff. corrected for Error in Dift.

Lat. in, corrected for Error in Dift.

40°20'N.
I 6 S.

47 14 N.

Agreeing exactly with my Latitude by Obfervation: I therefore conclude my Reckoning fufficiently correct. Then with the Dif ference of Latitude 66.2, and Departure 24.7, together with Yefterday's Latitude, I find the Difference of Longitude either by Middle Latitude or Mercator's Sailing.

In the laft Example 57.3, and 21.4, multiplied feverally by 156, thrice the measured Length of a Knot, and divide the two Products by 135, five Times the measured Time of the Glafs, will give the Difference of Latitude 66.2, and Departure 24.7, which is the fame Thing as if every Course had been corrected separately.

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Yefterday at Noon, we were in Lat. 36° 15' N. and have failed thefe 24 Hours S. E. E. 55 Miles, N. E. by N. 20 Miles, W. S. W. 70 Miles, S. by W. W. 20 Miles, and by Obfervation this Day at Noon we are in Lat. 34° 56′ N.

The TRAVERSE TABLE.

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By the Traverse Table it appears that by Account the Diff. of

Lat. is 64.2 S. and the Departure 16.9 W.

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1

I now examine the Log-line and Half-minute Glafs, and find them both right. Next I confider whether there be any Current, and I think I have Reason to suspect one: Upon Trial I find there is one fetting S. S. W. W. at the Rate of 7 Fathoms an Hour, and judge I have been in it these 24 Hours. Then 7 Fathoms (or Tenths of a Knot) per Hour, in 24 Hours, makes about 17 Miles; and to the Dift. 17 Miles, and Course S. S. W. & W. the Diff. of Lat. is 14.6 S. and Departure 8.7 W.

Diff. Lat. Dep. Now by Tra. Table 64.2S. 16.9W. And by Current 14.6S. 8.7W.

Correct for Current 78.8S. 25.6

Latitude failed from 36° 15'N.
Diff. of Lat. cor. for Cur. 1
19 S.

Lat. in correct for Cur. 34 56N,

Which agreeing with my Latitude by Obfervation, I conclude that my Reckoning is Right; then having the Latitude left, and Latitude come to, the Difference of Longitude may be found either by Middle Latitude or Mercator's Sailing as before.

If after all proper Allowances are made for Errors in Diftance, Currents, &c. the Latitude by Account and obferved Latitude should

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difagree, then the Reckoning must yet be further corrected; to do which, the following are the common, and feein to be the moft rational Methods.

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If the Courfe found by Dead Reckoning be less than 3 Points or 33 Degrees.

RULE. To the Difference of Latitude and Departure by Account find a Course; to this Courfe and the Difference of Latitude by Obfervation, find the Difference of Longitude, either by Middle Latitude or Mercator's Sailing.

EXAMPLE.

Yesterday, at Noon, we were in Lat. 39° 18' N. by an Obfervation, this Noon we are in Lat. 37° 48′ N. and our Dead Reckoning gives 107 Miles of Southing, and 64 of Wefting; required the true Difference of Longitude?

To the Difference of Latitude 107, and Departure 64, I find the Course 2 Points; then with the Meridional Difference of Latitude between the two Obfervations 115, and the fame Courfe, I find the true Difference of Longitude 69 Miles.

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If the Courfe found by Dead Reckoning be more than 3 Points, or 33°, and less than 5 Points, or 56°.

RULE. With the Diff. of Lat. and Dep. by Account, find the Diftance; with this Distance, and Diff. of Lat. by Obfervation, find another Departure. Take half the Sum of this Dep. and Dep. by Account, for the true Dep. with which and the Diff. of Lat. by Observation, find the Diff. of Long.

EXAMPLE.

Yefterday, at Noon, we were in Lat. 52° 40' N. and are this Noon in Lat. 54° 22' N. having by Account made 84 Miles of Northing and 76 Miles of Wefting; required the true Difference of Longitude?

To the Diff. of Lat. 84, and Dep. 76, the Distance is 113 Miles, and the Courfe 42°.

To Dift. 113, and Diff. of Lat. between the two Obfervations 102, the Dep. is 47.7; then 76 added to 47.7, is 123.7, half of which is 61.8, the true Dep.

To Dep. 61.8, and Diff. of Lat. by Obfervation 102, the Courfe is 31°, and with the Courfe 31°, and the Meridional Diff. of Lat. between the two Obfervations 171, I find the Diff. of Long. is 103 Miles.

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