530 



NA TURE 



[October 28, 1909 



the instrument is a Ritchie S-inch Uquid compass with the 

 card, however, removed, and an optical collimating system 

 with scale introduced, enabling the observer to note the 

 arc of motion of the magnet system while sighting on the 

 sun or star, hence knowing precisely to what part of the 

 .arc the stellar azimuth applies. In all forms of compass 

 .azimuth circles hitherto used, the magnetic azimuth of the 

 celestial body must be taken from whatever point the card 

 in its oscillations to and fro has momentarily reached. In 

 brief, practically the same method of observations can 

 now be used at sea as on land, where the magnetometer 

 circle would be set to some convenient point on the magnet 

 -scale and then scale readings taken of the positions of the 

 magnet during the interval of observations. The angle is 

 .next determined between the circle setting and some mark, 

 or the true meridian, and the declination is finally deduced. 

 Similarly with the marine collimator compass. The angle 

 .(say, middle of scale) between the magnet and some celestial 

 iody, as the sun, is read with a pocket se.xtant to the 

 nearest mijiute of arc at a given time, and then scale 

 .readings of magnet and of watch are taken. With the aid 

 of the time readings, the motion of the sun during the 

 Interval of observation is taken into account, and the true 

 .azimuths determined, whereas the scale readings give the 

 varying positions of the magnet system. 



With this instrument, therefore, one is almost entirely 

 independent of the yawing and rolling of the ship, making 

 it possible still to get satisfactory results when with all 

 other azimuth circle devices hitherto used at sea observa- 

 tions would be wholly impossible, or at least very uncertain. 

 In five to ten minutes a value of the magnetic declina- 

 tion is now obtained possessing an accuracy attainable 

 with previous instruments only by most careful observa- 

 tion and by laborious repetitions extending over a half-hour 

 or more to eliminate the motion of the card. Thus not 

 only has the accuracy of declinations at sea been in- 

 .creased, but, what is equally important, the time has been 

 reduced and the possibility of getting useful results in all 

 kinds of seas greatly extended. 



Furthermore, sufficient attention has not always been 

 paid in previous ocean magnetic work to a proper control 

 of constants. Thus, e.g., with the compass azimuth 

 circles, as usually constructed, there are movable parts 

 subject to wear, such as the axes of mirrors or of prisms 

 and of the azimuth circle on the bowl. The wearing of 

 these parts may casilv bring about the same effect as 

 though the compass were not mounted in the fore and aft 

 line, i.e. introduce a quantity .\ not due necessarily to the 

 ship's magnetism, but to instrumental error, which is likely 

 to vary with extent of use of azimuth device. To control 

 such errors, all instruments were invariably dismounted in 

 the case of the vessel (the Galilee) employed in the Pacific 

 Ocean work, whenever a port was reached, and correspond- 

 ing observations made between land and ship instruments. 

 With the present instrument, there being no such wearing 

 paits, there will not be the source of error described. 



It may also be pointed out that the effect of drag of 

 magnet system moving in the liquid during changes of the 

 skip's head is overcome in the present instrument, as well 

 as in the one mounted at C. for the method of observation 

 involves turning the compass bowl opposite to the ship's 

 niclion. 



At B is the gimbal stand for mounting an L.C. dip 

 •circle, as modified for the Galilee work, with which the 

 dip is determined absolutely (i.e. in all positions of circle 

 and needle, inclusive of reversal of polarity by an electric 

 coil) with two regular dip needles and again by the method 

 of deflections, and total intensity is obtained using two 

 deflecting distances. 



At C is a Ritchie liquid compass provided with an 

 improved azimuth circle and a deflecting attachment, both 

 designed and made by the Department of Terrestrial 

 Magnetism. With this instrument declinations are obtained, 

 and also the horizontal intensity of the earth's magnetic 

 ■force, by the method of deflections independently with two 

 deflecting magnets, the magnetic moments of which are 

 controlled from time to time by shore observations when- 

 ever the vessel is in port, and using two deflecting 

 ■distances. In these deflection observations the yawing of 

 the ship, or the changes in the lubber-line from which the 

 -deflection angles are counted, is controlled by the recorder 



N'O. 20S7, VOL. 81] 



stationed at D, who reads the ship's head by a spare 

 1 homson dry compass. The same kind of deflecting device 

 by which the deflecting magnet is brought at right angles 

 to the axis of the deflected card, thus admitting of the 

 simple sine-computation formula, is likewise attached to 

 the marine collimating compass at .\. In the latter case 

 the angle between deflected magnetic system and the sun 

 (or true meridian) is read with a pocket sextant, thus 

 making one entirely independent of the yawing of the 

 ship. 



The declinations obtained at C are intended chiefly as 

 some control against any gross blunders which may 'oe 

 made at A ; for a fairly smooth sea they compare favour- 

 ably with those got at A, but in rough seas the great 

 superiority of the A results is very evident. 



In brief, then, the scheme of observations, whenever 

 fully carried out, will yield the following determinations in 

 about one hour's time by independent observers, with different 

 instruments, and at different positions on the vessel : — 



At A. — Declinations (also horizontal intensity when a 

 celestial body is long enough visible to permit of full sets 

 of deflection observations). : 



.\t B. — Two values of dip by the regular absolute method, 

 and two values of total intensity and of dip, using two 

 deflecting distances. 



The horizontal intensity is determined by computation 

 from the dip and the total intensity. 



At C. — Two values of horizontal intensity independently 

 with two magnets, and using each time two deflecting 

 distances ; also, whenever possible, magnetic declinations. 

 D, as above described, is simply accessory to C, and does 

 not furnish any direct result. 



It is thus seen that an effective control is obtained for 

 each magnetic element, and it is for this purpose a great 

 gain indeed that it is now possible to compare at once the 

 values of horizontal intensity, for example, got at B and C 

 without first waiting until the deviations are well deter- 

 mined, as has hitherto been the case in all previous vessels 

 engaged in magnetic work — even on the Galilee, which, 

 before the Carnegie, had the smallest deviations of any 

 ship. 



Besides the great improvement resulting from having a 

 non-magnetic vessel, and from the perfection of the instru- 

 ments themselves, the conditions and opportunities for 

 observing have also been materially bettered on the 

 Carnegie. Thus the instruments at B and C are under 

 shelter, being mounted in observatories with revolving 

 domes and movable slides, permitting of both magnetic 

 and astronomical observations, with full protection to the 

 observer and instrument from w'ind and weather. 



Outstanding Difficulties. — These are chiefly due to 

 meteorological conditions and the state of the sea. Thus 

 in the absence of sun or star no magnetic declinations can 

 be obtained, though dip and intensities may be got even 

 in a pouring rain, because of the introduction of the 

 sheltering observatories. It is hoped that some instrument 

 based on the gyroscope compass may be soon perfected 

 having the desired accuracy, with the aid of which the 

 direction of the magnetic needle may be referred to an 

 invariable plane to be controlled whenever a celestial body 

 becomes visible. 



Were one to wait for a calm or a fairly smooth sea it 

 would occur quite frequently that no magnetic results 

 would be forthcoming. In fact, on the entire recent 

 October passage of the Carnegie from St. John's, New- 

 foundland, to Falmouth, England, there was not a single 

 day which would fall in the usual category of favourable 

 days of observation, but, instead, on nearly every day there 

 was a gale, the sea was rough, the vessel yawed through 

 an angle of io° or more, and rolled through an arc of 20° 

 to 30° and more, and yet observations were secured on 

 every day except one. That utilisable magnetic results have 

 still resulted under such very adverse conditions is due to 

 the perfection of the instruments, the cutting down of 

 time required for observations to get a desired degree of 

 accuracy, thus reducing to a minimum the condition of 

 steadiness of ship, and, of course, to the skill of the 

 observers. Still further improvements are being striven 

 for with regard to independence of steadiness of ship. 



It is thus seen that while the endeavour is steadfastly 

 held in mind to measure the magnetic elements at sea 



