March ii, 1920] 



NATURE 



^% 



only thing that these two instruments have in common 

 is the property of pointing north and south. I am 

 anxious that this should be clearly understood, because 

 in a recent lecture I gave at Bournemouth on this 

 very subject one of the audience asked me after the 

 lecture how the gyro-compass was shielded from out- 

 side magnetic influence. I pointed out, as I had 

 endeavoured to do during the lecture, that the gyro- 

 compass had nothing to do with magnetism, and, 

 therefore, did not require shielding. The magnetic 

 compass and the gyro-compass are, in fact, two 

 absolutely different instruments operated by entirely 

 different laws, although they are for the same purpose. 

 As many people do not understand why a gyro- 

 compass is needed when the magnetic compass is 

 already available, it is worth while to describe briefly 

 the magnetic or mariner's compass before attempting 

 to explain the gyro-compass. The mariner's compass 

 consists of a magnetic needle, or of several magnetic 

 needles fixed side by side, and balanced upon a sharp 

 point. A card divided into thirty-two (points of the 

 compass) is attached to the needle, and swings round 

 with it, so that the point marked N on the card 

 always points to the north. 



The earth, as we know, is a magnet, but not a 

 very powerful one, and it has been calculated that if 

 it were wholly of iron it would have an intensity of 

 magnetism 17,000 times greater than it has. All the 

 same, the magnetism is sufficiently strong to give a 

 t^'ood directive action to a pivoted needle. The mag- 

 netic poles of the earth are not coincident with the 

 geographical poles, but are situated some distance 

 away. The north magnetic pole was discovered by Sir 

 J. C. Ross to be situated in latitude 70^5' N. and longi- 

 tude 96° 46' W. in Boothia Felix, just within the Arctic 

 Circle some 1000 miles away from the actual pole. 



With this displacement of the magnetic poles we 

 have an irregular distribution of the magnetism over 

 the surface of the earth ; and thus the magnetic 

 needle does not point truly north and south at many 

 parts of the earth's surface. In London, for instance", 

 it points at an angle of 16° W. of the true north. 

 This angle is called the deviation or variation of the 

 needle. To enable ships to steer by the compass, 

 magnetic charts have been prepared and the deviation 

 at different places accurately measured. These mag- 

 netic charts have to be checked and altered from time 

 to time, as the deviation slowly varies from year to 

 year. Thus in London in 1659 the needle pointed true 

 north, while in 1820 there was an extreme westerly 

 variation of 24^°. Since then it has been slowly com- 

 ing back to something like 16° at the present time. 



On a wooden ship the accuracy of a good modern 

 magnetic compass leaves little to be desired, but on 

 an iron ship the case is quite different. The magnetic 

 field of the earth tends to be weakened in the length- 

 wise direction of the iron ship, because a portion of 

 the magnetism enters the ship, while across the sTiip 

 the field is stronger; and as it is essential that the 

 magnetism in which the needle lies should be uniform 

 in strength in whatever direction the ship may happen 

 to point, it is important that this stronger field should 

 be reduced by some method of magnetic shielding. This 

 is accomplished by fixing a pair of iron globes athwart 

 the ship on the two sides of the compass. The effect of 

 the iron of the ship and the corrections that have to 

 be made to the compass is to reduce the directive force 

 of the earth's magnetism, and thus the compass is 

 rendered slow and sluggish in its action. This is 

 partipularly the case on board a battleship. Tn the 

 interior of a submarine the- force is still further 

 reduced, so much so as to render the magnetic com- 

 pass useless for this class of vessel. 



Tt is quite oossible on an iron ship to correct the 

 NO. 2628, VOL. 105] 



errors of a compass, but as the ship itself may be a 

 magnet, and its strength a variable quantity, it is 

 important that the navigator should test the readings 

 of his compass at every available opportunity, and 

 particularly at the commencement of each voyage. 

 The ship's magnetism may quickly change through the 

 hammering action of the "waves, through the heating 

 action of the sun on one side of the vessel, or through 

 an earth on any of the electric wires that may be 

 running near the compass ; all these things together 

 add to the anxiety of the captain, as he is never quite 

 certain how far the compass is correct in its readings. 

 The swings of the modern compass are damped by 

 immersing the needles and card in a liquid such as 

 alcohol, but as this fluid" is attached to the ship and 

 turns with it, swinging the ship in any direction 

 carries the liquid round and reacts on the needle and 

 card, so that the compass has a tendency to be 

 carried round with the vessel. This lag in the instru- 

 ment renders it difficult to hold a ship dead on her 

 course, and the path, as a consequence, is sinuous, 

 and may oscillate, even in a calm sea, as much as 

 7° each side of the correct heading. As a shin has 

 usually to steam entirely by the readings of the 

 compass, any error is serious. For instance, if there 

 is an error of -^°, and the ship is steaming at sixteen 

 knots, she will move one English mile off her course 

 every hour. It is obvious how necessary it is to have 

 absolutely correct readings. 



Lord Kelvin was the first seriously to study the 

 errors of the magnetic compass. He started in 187 1, 

 and in 1876 produced his well-known instrument. 

 Although it was a great advance on any compass in 

 the British Navy, he had the greatest difficulty to get 

 it adopted; finally, in 1879 he proposed to place an 

 instrument at the disposal of the Admiralty at his 

 own expense. This offer was accepted. In spite of 

 this, it was only "through the acquaintance of influen- 

 tial naval officers, particularly of Capt. (now I^rd) 

 Fisher, that the compass was ever adopted. In 1880, 

 eighteen years after the commencement of his experi- 

 ments, and long after it was in common use in com- 

 mercial ships, he received official notification that his 

 lo-in. comoass was to be adopted in future as the 

 standard of the Nav\'. It is fortunate that we have 

 an alternative method of securing a north-seeking 

 property in the gvro-compass, an instrument of much 

 greater accuracy than the magnetic and with none 

 of its errors; for if deviations do occur they are 

 known deviations, and can therefore be allowed for. 



Evans and Smith, in 1861, were the first to discover 

 how important it was to mount the needles on the 

 card so that the moments of inertia of the moving 

 system should be the same about all directions — that 

 is to say that the system should be in dynamic 

 balance, otherwise the rolling of the ship would cause 

 deviations in the reading. I have lately discovered 

 that another deviation may be brought about, not by 

 an oscillation in one direction, but by the card being 

 set wobbling ; the needles and card would then have 

 a force applied trying to carry the moving system 

 round in the direction of the wobble. 



I have a magnetic compass here to demonstrate 

 this. It consists of a heavy brass disc mounted on a 

 vertical frictionless spindle. The needles are fixed to 

 the disc, and the whole movable system is carried on 

 a pendulous mounting, as in the irvro-compass. The 

 disc and needles are in correct static and dynamic 

 balance. Swiniring the pendulum in any one direction 

 produces no deviation, but by making it swim? in a 

 circular conical nath, thus giving a wobble tn the 

 plate, a serious deviation is caused in the rendim* of 

 the compass. The error is permanently maintained 

 against the earth's attraction so long as the circular 



