Mariners reckon by the civil Account of Time used 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. signifying after Mid-day; and the second 12 Hours with A. M. signifying 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 used 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 Persons with Regard to what deserves being recorded: Some approve of a Journal including the Log-book, each Day's Work at some Length, and such Occurrences as seem of most Importance; while others prefer a short Abstraćt of this long Journal, containing little more than the Course run, the Latitude and Longitude in, and sometimes the Bearing and Distance of the intended Port for each Day. In the following Journal the long Form is used as representing more fully each Day's Work, and the necessary Corrections; and an Abstraćt of this may be drawn out in the shortest Form that seems consistent with Distinctness. The Learner ought to be thoroughly acquainted with the long Form, and when he does that, he may either continue it, or take the shortest Form; or retrenching from the first, and adding to the second, what Particulars he thinks proper, and thereby make out a Form adapted to his own particular RULES Rules for correóting the DEAD RECKon ING by an Observation. OTWITHSTANDING the Rules already laid down forkeeping a Ship's Way at Sea, yet by Reason of the several Accidents that may attend a Ship in one Day's Run, such as swelling Seas, different Rates of sailing between the Times of heaving the Log, want of Care at the Helm in letting the Ship yaw or fall off, accidental Currents, sudden Squalls, when no Account can be kept, &c. the Latitude by Account and Latitude by Observation may very often differ, then it is necessary that proper Corrections be made in the Difference of Longitude. When you have made all proper Allowances you can, such as for Leeway, Variation, Currents, &c. and still find that your Latitude by Account will not agree with your Latitude by Observation, then you must correct as follows: First, Consider 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 just, and the Log properly hove, or any sudden Squalls, or proper Allowances made for the Leeway, &c. which of these you conjećture our Error is in ; make what Allowances you think meet to your {j of Latitude and Departure by Dead Reckoning, and see if that will reform your Latitude by Account, so as to make it agree with your Latitude by Observation; if it does, you have guessed right; (for you must always keep to the Latitude by Observation, 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 Mistakes in your Conječture, or some other Cause which produces the Error in the Reckoning, and stands in need of being correóted. In this Case, you are first to examine your Log-line and Half-minute Glass, and if there be an Error in them, allow for it, as in the following Example. 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 48° 20' N. Differing 9 Miles from the true Latitude by Observation. Wherefore I examine the Log-line and Half-minute Glass, 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 Case III. and find my correct Difference of Latitude 66.2. S. and my Departure 24.7 W. Agreeing exačtly with my Latitude by Observation: I therefore conclude my Reckoning sufficiently correct. Then with the Difference of Latitude 66.2, and Departure 24.7, together with Yesterday’s Latitude, I find the Difference of Longitude either by Middle Latitude or Mercator’s Sailing. In the last Example 57.3, and 21.4, multiplied severally by 156, thrice the measured Length of a Knot, and divide the two Products by 135, five Times the measured Time of the Glass, will give the Difference of Latitude 66.2, and Departure 24.7, which is the same Thing as if every Course had been corrected separately. By the Traverse Table it appears that by Account the Diff. of Lat. is 64.2 S. and the Departure 16.9 W. Latitude sailed from 36° 15' N. Which agreeing with my Latitude by Observation, 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 Distance, Currents, &c. the Latitude by Account and observed Latitude should - i. disagree, |