Dead Reckoning and Astronomical Observation.

Otago Witness, Issue 1917, 17 August 1888, Page 32

Dead Reckoning and Astronomical Observation.

As observation may be prevented by thick weather, the second method may fail throughout a voyage, or during the critical parts of it when land is approached. This dead reckoning, on which in many cases the most scientific captains may have to trust, must be considered first. Dead reckoning (why called " dead " I dc not know nor greatly care) depends on the determination hour by hour of the direction and velocity of the ship's motion, and the determination thence of her position after each day's various movements. If we know a ship's position at a given time — say at noon on July I—and1 — and she sails or steams at a known rate in a determined direction for a certain time — an hour say — we can calculate her position at the end of that hour, subject to the action of currents. Since the rate and direction of currents are mostly known we can take this particular cause of motion into account. Thus we can mark in on our chart the position of the ship at the end of the hour. So at the end of the next interval of time during which her course remains unchanged and so forth — till fiually we determine her position at noon on July 2. (This is not necessarily 24 hours from noon July L, as in the new position now attained there i 3 different local time, earlier i£ the ship is west of her former position later if she is east of it.) Going on in this way day after day, the ship's course can be charted in, and her position is known fronx day to day — or in other words those who have her in charge thus find their way from port to port. Theoretically thi3 is all right, practically there are difficulties. The circumstances which render "dead reckon-

ing " an unsatisfactory method of navigation are unfortunately numerous. The determination of the '"ship's course depends on the indications of