POLE TO POLE

By GEOFFREY RAWSON HE air reconnaissance of the North Magnetic Pole last month, and the report that it had moved some distance from its position as defined a century ago, seems to have elicited little public interest. Even the physicists and others primarily concerned do not seeds to be greatly interested. As for the navigators, to whom the Poles have been the most important factor in navigation for centuries, they too seem to be losing all interest in the whole subject of Magnetism. The truth is, of course, that that ancient, complicated and unreliable Instrument known as the magnetic compass is passing into obsolesc- ence after its long reign. It,has had a notable, if erratic place in discovery, exploration, survey and charting. Columbus steered west with its aid and even seems to have been acquainted with its varia-

tion, namely the angle between. the magnetic and the geographic poles. In those days, some even thought that the longitude itself

could be deduced in some mysterious way from a knowledge of the magnetic variation. In the course of time, the crude needle was im- proved and compensated for all kinds of inherent and extraneous

errors ; and finally, in the last century, a noted Scots scientist, Lord Kelvin, produced his .greafly improved standard model, with its eight parallel needles, its featherlight card, its beautiful compensations and its general efficiency and reliability.

Even so, the mariners' compass remained suspect. No navigator- ever felt safe with it; the worst insult at sea was to "lie like a Magnetic compass "; it seldom, if ever, performed its true function—to point

to the true north; it frequently pointed in the opposite direction, its error varied from one degree -to ninety; it had to be continually checked day and night by amplitudes and azimuths; in warships, gunfire abnormally disturbed it. Furthermore, the nearer you approached one or other of the magnetic poles, the more erratic the magnetic compass. The aircraft which recently flew over the North

.Magnetic Pole had on board no fewer than- eleven magnetic‘..com.

passes. The-captain, Wing -Commander-D. C. McKinley,- is-reported to have said wryly : "they did not seem to know what to do when we were over the Pole." It was clear that a revolution was long overdue with the coming of high-speed- liners, bigger and 'better battleships and fast aircraft. The scientists worked on the problem and produced the Sperry gyroscopic compass. It had all the virtues

and none GU-the vices of the old instrument; -it pointed to the true north; it loftily ignored even the existence- of -the magnetic poles; it

completely eliminated Variation and, Deviation and it was immtine altogether -to magnetic influences.. Hence the general jndifference to the news that the Magnetic Pole -is not where it was thought to be. Who cares? Hence also the-passing-of-the-first great aid to navigation --the magnetic corimass.

The navigator not only :wanted to know-in-Which direction he was steering; it was essential that he should know the depth of water into which he was steering. From the dawn of time he had relied for this purpose on a weight at the end of a piece of rope or wire. This was " cast " Overboard and then slowly and painfully hauled back on board and the depth in fathoms laboriously measured by -an ancient systlem or code of knots and bits of rag and other queer marks. I well -remem.ber, not so many years ago. taking part in the nautical operation of "casting the deep sea lead." It was usually necessary to do this on a dark and dirty night when the ship was approaching the land and the-captain was not sure where he was and the .temperature was low(icy, in fact) and the mainyard had to be hacked. The whole watch was employed lining _the bulwarks, and as the leadsman cast the lead, his ghostly call in the darkness from the bows "Watch, there, Watch! " still haunts my ears.

This primitive method persisted, with slight technical improve- ments, from before the-early Phoenicians to after the late Victorians. Then scien2e, appalled, by the age-old monstrosity and its hopeless inadequacy to present-day needs, took a hand. There resulted the sonic depth-finder by which a sound wave is continuously transmitted to the-Sea bed from which its echo returns to a receiving diaphragm on board the -ship. The speed of- sound in sea water being known, the depth is automatically and continuously registered before the grati- fied eye of the navigator in the secluded, comfort and convenience of the chart room. It not only assures him of the ship's safety as she

speeds on but gives him a continuous line of soundings for com- parison with the lines of soundings on the chart before him.

So passes the second great aid to navigation—the sounding lead.

In addition to knowing his course and direction and the depth of water under his keel, it is essential that the navigator should know the position of his ship at any given time, particularly when he is out of sight of land and seamarks. Position at sea means Latitude plus Longitude. Latitude has always been easy and simple. Even Old Man Columbus had a pretty shrewd idea of his latitude, and heaven knows he was poorly enough equipped. But finding the longitude was the devil, because longitude is the same as time, and in medieval times, and still later, nobody had yet been able to establish any method or medium of knowing the correct time in a ship at sea. Ships found out where they were by hitting the rocks and even then they did not know the longitude of the rock. So charts could not be made; voyages were unduly prolonged; shipmasters went a long way round to avoid suspected dangers, and at night fall it was the general custom to heave-to for the night.

Things got so bad that at last the British Government was goaded into offering some handsome prizes. By an Act of Parliament passed in the reign of Queen Anne, they offered a prize of L20,000 to any- body who could discover a method of finding the longitude of a ship at sea. This was in 1714. Half a century later, the prize was grudg- ingly awarded to a Yorkshire carpenter, John Harrison, who aban- doned carpentry to enter on his life's work, which was the building of the perfect chronometer. For this vocation he had a peculiar bent. He was a genius in horology.

His masterpiece maintained such a regular rate, even during a long passage in a ship at sea, that the correct longitude was easily found by its means to within a very small margin of error. ,

. The reason was that longitude depends on Greenwich mean rime, and Harrison's chronometer, day after day, did not vary, or varied so little that it did not matter, and thus gave the navigatot. the correct Greenwich Time. This chronometer was duplicated and multiplied by dozens' of other watchmakers, and during the: ath, 19th and the early part of the zoth century every 'ship was navigated with the aid of one or more of these beautiful instruments, all facsimiles of Harrison's master watch. Then came radio. And radio brought daily time signals from all the principal Observatories of the world. It did not matter if you were at the North Magnetic Pole or in the African jungle, provided you had a radio receiver, you could get G.M.T. at any hour. At sea, it was found that a common deck watch sufficed. Harrison's beautiful instrument installed in every ship became

redundant. So passed the third great aid to navigation—the marine . „ chronometer.

There passed also something else—the need of the navigator even to know Greenwich Time. With the coming of Radar, the longitude of a ship at sea is becoming of dwindling importance. The ship of the future will move along the Radar Ray, guided and controlled on its .set path. Longitude will be of little consequence. Time, even, will cease to be of much account in navigation. Then perhaps he navigator will find his occupation gone.