The Magnetic Needle

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

The Magnetic Needle

Of course everyone knows that the needle does not point due north. " True as the needle to the pole" may do very well for poetry, but it is about as far from truth as ifc can well be. The needle not only does not only point to the pole in 999 places out of 1000 ; it does not point in the same degree or way from the true north at different places ; nay, its direction is not the same at the same place at different times. There are parts of the United States where the needle points west of north, parts where it points east of north. In England the needle points 20deg west of north (nearly N.N.W.) ; 20 years -ago the needle pointed farther west; two centuries ago the needle in England pointed nearly north ; three centuries ago it pointed 20deg east of north, (nearly N.N.E.). Observers can keep account of these mysterious changes at fixed stations ; but although our physical geographies give the magnetic meridians for the whole earth at given times, the position of these meridians is not accurately known for large parts of the ocean (and even of continents) at any time, and the changes taking place are far from being accurately determined either as to nature or as to rate. Then the magnetic needle in each ship has its own special peculiarities. In different positions of the ship, the deviation of the ship's needle from the true position (for the place where she is) is different. If she has been "swung" under proper conditions before starting, the different deviations for different positions may be determined ; but sometimes blows received at sea, great changes of temperature, and similar causes, so far affect what may be called the ship's magnetism that the deviations aie changed from those noted in port ; and in such degree the indications 6f the needle are thenceforth unsatisfactory. Then further as to direction, it is by no means an easy thing for a steersman to keep the ship's course exactly to that desired. To begin with the compass card is small, and a deviation from the direction which would make an important difference in a run of a mile would be almost imperceptible on the card.. But apart from this it takes a good steersman to keep a ship steady on any course. The momentary deviations this side and that may be slight, but in the long run a measurable deviation from the mean course may arise, which must be regarded as a distinct deviation from che intended track. Lastly (as regards direction of motion) ' every ship makes more or less leeway — that is, besides advancing in the direction of her length, the only part of her advance of which the needle tells, she is carried sideways through the water — unless the wind is directly astern of her. Then as to rate of motion, the log affords the only means available for