ASSISTING THE NAVIGATOR.

Press, Volume XLVIII, Issue 14525, 29 November 1912, Page 2

ASSISTING THE NAVIGATOR.

BRITISH ANTARCTIC EXPEDITION.

IMPORTANT MAGNETIC OBSERVATIONS.

One of tho important branches of scientific research undertaken by Captain Scott's Antarctic Expedition, is the magnetic observation work, which i 3 being carried out by the officers of the Itoyal Navy who are members of the expedition, and who possess special qualifications for the work. Tho cruise of the Terra Nova outside Lytte'.ton Heads on Tuesday was undertaken for the purpose of obtaining important data for use in connection with the magnetic work of tho expedition. The movements of tho vessel during her nine hours' cruiso were puzzling enough to tho landsman, for during that time sho steamed round in many circles, or moved inshore and offshore, steering courses on many different points of the compass. Next to the officers who were engaged in making observations with the compasses and other instruments on the observing station amidships, tho hard-est-workod men on deck' were the seamen, who in their spoils at the wheel were constantly changing the ship's course in iespouse to tho observing officers' orders.

Lieut. 11. L. L. Pennell, R.N., who was in charge of the magnetic observations, gave an interesting explanation of tho day's work, to ' the representative of ''Tho Press, , ' who was on board tho Terra Nova during her cruise on Tuesday.

Lieut. Pennell said that in order to understand the use of the magnetic work done in a ship such as the lerra Nova, and its practical application, it was necessary to remember that— (1) The magnetic needle always lies (when freely suspended) at an to the True Meridian, that is, it does not point towards the true Pole of the earth, but towards the Magnetic Pole. This angle is called tho variation, and is, of course, different in dilierent parts of the earth's surface. (2) The needle is also inclined at an angle to the horizontal, which is called its dii>.

(3) The actual forco holding tho needle is called the total magnetic force.

Both "dip" and "force" vary in different parts of the world. Not only do the variation dip and force vary for different positions of the earth s surface, but all three have a changing valuo for any particular place, bo that, in tho courso of years, any ono place completely alters its magnetic elements.

As an example, Lieut. Pennell quoted the variation of London, as given in tho following table: — Year. Variation. 1580 11 degrees 17 minutes East. 1657 Nil. 1819 24 degrees 41 minutes West

maximum. 1909 16 degrees 16 minutes West,

i and as regards the dip: — Year. Dip. 1720 75 degrees (tho maximum reached, and only 16 degrees from being absolutely vertical). ( 1900 67 degrees. VALUE TO NAVIGATION. It was therefore of supremo importance for a navigator .to know tho variation wherever his ship might be, so as to know what course to steer to reach his destination, while the Dip and Total Magnetio Force were very closely connected with tho orror caused to his compass in any place, by the iroii in his own sliip. The Admiralty publish charts showing the variation, dip and total forco all ovor tho world, but, as has been stated, these elements are always undergoing secular change, and to keep the charts correct, fresh information has to bo constantly suppiied. In the North, the secular chango can be forecastod ivith very fair accuracy, as the oceans are surrounded by civilised lands having numerous magnetic observatories; but in the South, data as to the great Soulihern oceans is very meagre. Lieutenant Pennell explained that in a ship such as the Terra Nova, that wished to dotermin.o tho maguetio elements, the difficulty aroso that the ship herself had much iron in her structure, for instance, tho engines, boiler, funnel, and tho metal lining in I her provision stores, and this iron affected the position from which the observations woro made. It Avas, theroforej necessary to find out what corrections had to bo allowed for the phip's iron, to correct tho observations as actually taken, and it was for this purpose that tho vessel's cruise outside Lyttelton Heads was made on Tuesday. To find the corrections, magnetised needles wore vibrated horizontally and vertically in the observing position in the chip (namely, on top of the icehouso amidships), with the ship's head pointing nortn, south, cast and west, and the timo the needles took to vibrato was compared with tho timo they took on chore. This work was carried out in New Zealand waters, with Lyttelton as tho station, and again in the Antarctic. Tho final corrections would bo obtained by a rather long mathematical process. To determine the variation, tho ship's I head was put on each of the eight | principal points of tho compass (where possiblo on sixteen points), and tho error of the compass was determined on each point by an observation of tho sun (the moon or stars can be used at nipent). The mean raluo of tho eight or eixtcon observations was tho correct variation.

It would be seen, therefore, explained Lieut. Pennell. that the deviation was obtained at tho same time as the variation. Tho deviation was the orror of tho compass duo to the iron in the ship, and was different on every point of tho compass tho ship's head was put on, so that for any point it was tho differenoo between tho error observed on that point and tlie mean of the swing. .It would be seen, therefore, that the magnetic work of the Expedition had a very practical bearing ,as far as shipping and navigation was concerned.