2. Departure taken from St. Agnes Lighthouse, Scilly Islands, Lat. 49° 53' S, Lo. 6° 20' W, bearing by compass E x S, distance 18 knots, Deviation 10° W, Variation 23° W. Ship headed N steamed on the following courses:
Assign for Night Work the following articles in Bowditch: 179-180-181-182. Also additional problems in Dead Reckoning.
FRIDAY LECTURE
Mercator Sailing
This is a method to find the true course and distance between two points. The method can be used in two ways, i.e., by the use of Tables 2 and 3 (called the inspection method) and by the use of logarithms. The first method is the quicker and will do for short distances. The second method, however, is more accurate in all cases, and particularly where the distances are great. The inspection method is as follows (Put in your Note-Book):
Find the algebraic difference between the meridional parts corresponding to the Lat. in and Lat. sought by Table 3. Call this Meridional difference of Latitude. Find the algebraic difference between Longitude in and Longitude sought and call this difference of Longitude. With the Meridional difference of Latitude and the difference of Longitude, find the course by searching in Table 2 for the page where they stand opposite each other in the latitude and departure columns. Now find the real difference of latitude. Under the course just found and opposite the real difference of Latitude, will be found the distance sailed in the distance column. Example:
What is the course and distance from Lat. 40° 28' N, Lo. 73° 50' W, to Lat. 39° 51' N, Lo. 72° 45' W?
| Lat. in | 40° 28' N | Meridional pts. | 2644.2 |
| Lat. sought | 39 51 N | Meridional pts. | 2596.0 |
| 0° 37' | Mer. diff. Lat. | 48.2 | |
| Lo. in | 73° 50' W | ||
| Lo. sought | 72 45 W | ||
| 1° 05' = 65' |