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TERRESTRIAL MAGNETISM
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TERRESTRIAL See also:MAGNETISM , the See also:science which has for its See also:province the study of the magnetic phenomena of the See also:earth . § 1 . Terrestrial See also:magnetism has a See also:long See also:history . Its See also:early growth was slow, and considerable uncertainty prevails as to its earliest developments . The properties of the magnet See also:Historical . (see MAGNETISM) were to some small extent known to the Greeks and See also:Romans before the See also:Christian era, and compasses (see See also:COMPASS) of an elementary See also:character seem to have been employed in See also:Europe at least as early as the 12th See also:century . In See also:China and See also:Japan compasses of a See also:kind seem to have existed at a much earlier date, and it is even claimed that the See also:Chinese were aware of the See also:declination of the compass See also:needle from the true See also:north before the end of the 11th century . Early scientific know-ledge was usually, however, a mixture of facts, very imperfectly ascertained, with philosophical imaginings . When an early writer makes a statement which to a See also:modern reader suggests a knowledge of the declination of the compass, he may have had no such definite See also:idea in his mind . So far as Western See also:civilization is concerned, See also:Columbus is usually credited with the See also:discovery—in 1492 during his first voyage to See also:America—that the pointing of the compass needle to the true north represents an exceptional See also:state of matters, and that a declination in See also:general exists, varying from See also:place to place . The See also:credit of these discoveries is not, how-ever, universally conceded to Columbus . G .
Hellmann 6 * considers it almost certain that the departure of the needle from the true north was known in Europe before the See also:time of Columbus
.
There is indirect See also:evidence that the declination of the compass was not known in Europe in the early See also:part of the 15th century, through the peculiarities shown by early maps believed to have been See also:drawn solely by regard to the compass
.
Whether Columbus was the first to observe the declination or not, his date is at least approximately that of its discovery
.
The next fundamental discovery is usually ascribed to See also:Robert See also:Norman, an See also:English See also:instrument maker
.
In The Newe Attractive (1581) Norman describes his discovery made some years before of the inclination or See also:dip
.
The discovery was made more or less by See also:accident, through Norman's noticing that compass needles which were truly balanced so as to be See also:horizontal when unmagnetized, ceased to be so after being stroked with a magnet
.
Norman devised a See also:form of dip-circle, and found a value for the inclination in See also:London which was at least not very wide of the See also:mark
.
Another fundamental discovery, that of the See also:secular See also:change of the declination, was made in See also:England by See also:
It was not until midsummer 1634 that Gellibrand felt sure of his facts, and yet the change of declination since 158o exceeded 7° The delay probably arose from the strength of the preconceived idea, apparently universally held, that the declination was absolutely fixed
.
This idea, it would appear, derived some of its strength from the See also:positive assertion made on the point by See also: He also found that the See also:amplitude of the movement was considerably larger in summer than in See also:winter . Canton further observed that in a few days the movements were conspicuously irregular, and that See also:aurora was then visible . This association of magnetic disturbance and aurora had, however, been observed somewhat before this time, a description of one conspicuous instance being contributed to the Royal Society in 1750 by Pehr Vilhelm Wargentin (1717–1783), a Swede . Another landmark in the history of terrestrial magnetism was the discovery towards the end of the 18th century that the intensity of the resultant magnetic force varies at different parts of the earth . The first observations clearly showing this seem to be those of a Frenchman, See also:Paul de Lamanon, who observed in 1785–1787 at See also:Teneriffe and See also:Macao, but his results were not published at the time . The first published observations seem to be those made by the See also:great traveller See also:Humboldt in tropical America between 1998 and 1803 . The delay in this discovery may again be attributed to instrumental imperfections . The method first devised for comparing the force at different places consisted in taking the time of oscillation of the dipping needle, and even with modern circles this is hardly a method of high precision . Another discovery See also:worth chronicling was made by See also:Arago in 1827 . From observations made at See also:Paris he found that the inclination of the dipping needle and the intensity of the horizontal component of the magnetic force both possessed a diurnal variation . § 2 . Whilst See also:Italy, England and See also:France claim most of the early observational discoveries, See also:Germany deserves a large See also:share of credit for the great improvement in See also:instruments and methods during the first See also:half of the 19th century . Measurements of the intensity of the magnetic force were somewhat crude until See also:Gauss showed how See also:absolute results could be obtained, and not merely relative data based on observations with some particular needle . Gauss also devised the bifilar See also:magnetometer, which is still largely represented in instruments measuring changes of the horizontal force; but much of the See also:practical success attending the application of his ideas to instruments seems due to Johann von See also:Lamont (1805–1879), a Jesuit of Scottish origin See also:resident in Germany . The institution of See also:special observatories for magnetic See also:work is largely due to Humboldt and Gauss . The latter's See also:observatory at See also:Gottingen, where regular observations began in 1834, was the centre of the Magnetic See also:Union founded by Gauss and See also:Weber for the carrying out of simultaneous magnetic observations and it was long customary to employ Gottingen time in schemes of See also:international co-operation . In the next See also:decade, mainly through the See also:influence of See also:Sir See also:Edward See also:Sabine (1788–1883), afterwards See also:president of the Royal Society, several magnetic observatories were established in the See also:British colonies, at St See also:Helena, Cape of See also:Good See also:Hope, Hobarton (now See also:Hobart) and See also:Toronto . These, with the exception of Toronto, continued in full See also:action for only a few years; but their records—from their widely distributed positions—threw much fresh See also:light on the differences between magnetic phenomena in different regions of the globe . The introduction of regular magnetic observatories led ere long to the discovery that there are notable differences between the amplitudes of the regular daily changes and the frequency of magnetic disturbances in different years . The discovery that magnetic phenomena have a See also:period closely similar to, if not absolutely identical with, the " eleven See also:year " period in sunspots, was made independently and nearly simultaneously about the See also:middle of the 19th century by Lamont, Sabine and R . See also:Wolf . The last half of the 19th century showed a large increase in the number of observatories taking magnetic observations . After 1890 there was an increased See also:interest in magnetic work . One of the contributory causes was the magnetic survey of the British Isles made by Sir A .
Rucker and Sir T
.
E
.
See also:Thorpe, which served as a stimulus to similar work elsewhere; another was the institution by L
.
A
.
See also:Bauer of a See also:magazine, Terrestrial Magnetism, specially devoted to the subject
.
This increased activity added largely to the stock of See also:information, sometimes in forms of marked practical utility; it was also manifested in the publication of a number of papers of a speculative character
.
For historical details the writer is largely indebted to the See also:works of E
.
See also: The unifilar magnetometer (q.v.), for tionat instance, gives the absolute values of the declination and Methods and horizontal force, whilst the See also:inclinometer (q.v.) or dip Records. circle gives the inclination of the dipping needle . Instruments of the second kind, termed magnetographs (q.v.), are See also:differential and self-recording, and show the changes constantly taking place in the magnetic elements . The See also:ordinary form of See also:magnetograph records photographically . Light reflected from a fixed See also:mirror gives a See also:base See also:line answering to a See also:constant value of the See also:element in question; the light is cut off every See also:hour or second hour so that the base line also serves to make the time . Light reflected from a mirror carried by a magnet gives a curved line answering to the changes in position of the magnet . The length of the See also:ordinate or perpendicular drawn from any point of the curved line on to the base line is proportional to the extent of departure of the magnet from a See also:standard position . If then we know the absolute value of the element which corresponds to the base line, and the See also:equivalent of r cm. of ordinate, we can deduce the absolute value of the element answering to any given instant of time . In the See also:case of the declination the value of 1 cm. of ordinate is usually dependent almost entirely on the distance of the mirror carried by the magnet from the photographic See also:paper, and so remains invariable or very nearly so . In the case of the horizontal force and See also:vertical force magnetographs—these being the two force components usually recorded—the value of 1 cm. of ordinate alters with the strength of the magnet . It has thus to be determined from time to time by observing the deflection shown on the photographic paper when an See also:auxiliary magnet of known moment, at a measured distance, deflects the magnetograph magnet . Means are provided for altering the sensitiveness, for instance, by changing the effective distance in the bifilar suspension of the horizontal force magnet, and by altering the height of a small See also:weight carried by the vertical force magnet . It is customary to aim at keeping the sensitiveness as constant as possible . A very See also:common standard is to have 1 cm. of ordinate corresponding to 1o' of arc in the declination and to soy (1y.o 0000I C.G.S.) in the horizontal and vertical force magneto-graphs . As an example of how the curves are standardized, suppose that absolute observations of declination are taken four times a See also:month, and that in a given month the mean of the observed values is 16° 34'•6 W . The curves are measured at the places which correspond to the times of the four observations, and the mean length of the four ordinates is, let us say, 2.52 cros . If cm. answers to 1o', then 2.52 cms. represents 25'•2, and thus the value of the base line—i.e. the value which the declination would have if the See also:curve came down to the base line—is for the month in question 16° 34'•6 less 25'•2 or 16° 9' 4, If now we wish to know the declination at any instant in this particular month all we have to do is to measure the corresponding ordinate and add its value, at the See also:rate of to' per crn., to the base value 16° 9'•4 just found . Matters are a little more complicated iii the case of the horizontal and vertical force magnetographs . Both instruments usually possess a sensible temperature coefficient, i.e. the position of the magnet is dependent to some extent on the temperature it happens to possess, and See also:allowance has thus to be made for the difference from a standard temperature . In the case of the vertical force an " observed " value is derived by combining the observed value of the inclination with the simultaneous value of the horizontal force derived from the horizontal force magneto-graph after the base value of the latter has been determined . In themselves the results of the absolute observations are of See also:minor interest . Their See also:main importance is that they provide the means of fixing the value of the base line in the curves . Unless they are made carefully and sufficiently often the information derivable from the curves suffers in accuracy, especially that See also:relating to the secular change . It is from the curves that information is derived as to the regular diurnal variation and irregular changes . In some observatories it is customary to publish a See also:complete See also:record of the values of the magnetic elements at every hour for each day of the year . A useful and not unusual addition to this is a statement of the absolutely largest and smallest values of each element recorded during each day, with the precise times of their occurrence . On days of large disturbance even hourly readings give but a very imperfect idea of the phenomena, and it is customary at some observatories, e.g . See also:Greenwich, to reproduce the more disturbed curves in the See also:annual See also:volume . In calculating the regular diurnal variation it is usual to consider each month separately . So far as is known at See also:present, it is entirely or almost entirely a See also:matter of accident at what precise hours specially high or See also:low values of an element may present themselves during an individual highly disturbed day; whilst the range of the element on such a day may be 5, to or even 20 times as large as on the average undisturbed day of the month . It is thus customary when calculating diurnal in-equalities to omit the days of largest disturbance, as their inclusion would introduce too large an element of uncertainty . Highly disturbed days are more than usually common in some years, and in some months of the year, thus their omission may produce effects other than that intended . Even on days of lesser disturbance difficulties present themselves . There may be to and fro move-merits of considerable amplitude occupying under an hour, and the hour may come exactly at the See also:crest or at the very lowest part of the trough . Thus, if the See also:reading represents in every case the ordinate at the precise hour a considerable element of See also:chance may be introduced . If one is dealing with a mean from several See also:hundred days such " accidents " can be trusted to practically neutralize one another, but this is much less fully the case when the period is as See also:short as a month . To meet this difficulty it is customary at some observatories to derive hourly values from a freehand curve of continuous curvature, drawn so as to smooth out the apparently irregular movements . Instead of See also:drawing a freehand curve it has been proposed to use a planimeter, and to accept as the hourly value of the ordinate the mean derived from a See also:consideration of the See also:area included between the curve, the base line and ordinates at the See also:thirty minutes before and after each hour . § 4 . Partly on account of the uncertainties due to disturbances, and partly with a view to See also:economy of labour, it has been the practice at some observatories to derive diurnal inequalities from a comparatively small number of undisturbed or quiet days . Beginning with 189o, five days a month were selected at Greenwich by the astronomer royal as conspicuously quiet . In the selection regard was paid to the desirability that the See also:arithmetic mean of the five See also:dates should See also:answer to near the middle of the month . In some of the other English observatories the routine measurement of the curves was limited to these selected quiet days . At Greenwich itself diurnal inequalities were derived regularly from the quiet days alone and also from all the days of the month, excluding those of large disturbance . If a quiet day differed from an ordinary day only in that the diurnal variation in the latter was partly obscured by irregular disturbances, then supposing enough days taken to smooth out irregularities, one would get the same diurnal inequality from ordinary and from quiet days . It was found, however, that this was hardly ever the case (see §§ 29 and 30) . The quiet day See also:scheme thus failed to secure exactly what was originally aimed at ; on the other See also:hand, it led to the discovery of a number of interesting results calculated to throw valuable sidelights on the phenomena of terrestrial magnetism . The idea of selecting quiet days seems due originally to H . See also:Wild . His selected quiet days for St See also:Petersburg and See also:Pavlovsk were very few in number, in some months not even a single day reaching his standard of freedom from disturbance . In later years the International Magnetic See also:Committee requested the authorities of each observatory to arrange the days of each month in three See also:groups representing the quiet, the moderately disturbed and the highly disturbed . The See also:statistics are collected and published on behalf of the committee, the first to undertake the See also:duty being M . Snellen . The days are in all cases counted from Greenwich See also:mid-See also:night, so that the results are strictly synchronous . The results promise to be of much interest . § 5 . The intensity and direction of the resultant magnetic force at a spot—i.e. the force experienced by a unit magnetic pole—are known if we know the three components of force parallel to any set of orthogonal axes . It is usual to take for these axes the vertical at the spot and two perpendicular axes in the horizontal See also:plane; the latter are usually taken in and perpendicular to the See also:geographical See also:meridian . The usual notation in mathematical work is X to the north, Y to the See also:west or east, and Z vertically downwards . The international magnetic committee have recommended that Y be taken positive to the east, but the fact that the declination is westerly over most of Europe has often led to the opposite See also:procedure, and writers are not always as careful as they should be in stating their choice . Apart from mathematical calculations, the more usual course is to define the force by its horizontal and vertical components —usually termed H and V—and by the declination or See also:angle which the horizontal component makes with the astronomical meridian . The declination is sometimes counted from o° to 36o°, o° answering to the case when the so-called north pole (or north seeking pole) is directed towards geographical north, 9o° to the case when it is directed to the east, and so on . It is more usual, however, to reckon declination only from o° to 18o°, characterizing it as easterly or westerly according as the north pole points to the east or to the west of the geographical meridian . The force is also completely defined by H or V, together with D the declination, and I the inclination to the See also:horizon of the dipping needle . Instead of H and D some writers make use of N the northerly component, and W the westerly (or E the easterly) . The resultant force itself is denoted sometimes by R, sometimes by T (See also:total force) . The following relationships exist between the symbols X=N, Y=W or E, Z=V, R=T, H— V (X2-+2), Rse (X2+Y2+Z2), tan D = Y/X, tan I = V/H . The See also:term magnetic element is applied to R or any of the components, and even to the angles D and I . § 6 . Declination is the element concerning which our know-ledge is most complete, and most reliable . With a good unifilar magnetometer, at a fixed observatory distant charts. from the magnetic poles, having a fixed mark of known See also:azimuth, the observational uncertainty in a single observation should not exceed o'•5 or at most r'.o . It cannot be taken for granted that different unifilars, even by the best makers, will give absolutely identical values for the declination, but as a matter of fact the differences observed are usually very trifling . The See also:chief source of uncertainty in the observation lies in the torsion of the suspension fibre, usually of See also:silk or more rarely of phosphor See also:bronze or other See also:metal . A very stout suspension must be avoided at all cost, but the fibre must not be so thin as to have a considerable See also:risk of breaking even in skilled hands . Near a magnetic pole the directive force on the declination magnet is reduced, and the effects of torsion are correspondingly increased . On the other hand, the regular and irregular changes of declination are much enhanced . If an observation consisting of four readings of declination occupies twelve minutes, the chances are that in this time the range at an English station will not exceed 1', whereas at an See also:arctic or. See also:antarctic station it will frequently exceed ro' . Much greater uncertainty thus attaches to declination results in the Arctic and Antarctic than to those in temperate latitudes . In the case of secular change data one important consideration is that the observations should be taken at an absolutely fixed spot, See also:free from any artificial source of disturbance . In the case of many of the older observations of which records exist, the precise spot cannot be very exactly fixed, and not infrequently the site has become unsuitable through the erection of buildings not free from See also:iron . Apart from buildings, much depends on whether the.neighbourhood is free from basal-See also:tic and other magnetic rocks . If there are no See also:local disturbances of this sort, a few yards difference is usually - without appreciable influence, and even a few See also:miles difference is of minor importance when one is calculating the mean secular change for a long period of years . When, however, local disturbances exist, even a few feet difference in the site may be important, and in the See also:absence of positive knowledge to the contrary it is only prudent to See also:act as if the site were disturbed . Near a magnetic pole the declination naturally changes very rapidly when one travels in the direction perpendicular to the lines of equal declination, so that the exact position of the site of observation is there of special importance . The usual method of conveying information as to the value of the declination at different parts of the earth's See also:surface is to draw curves on a See also:map—the so-called isogonals—such that at all points on any one curve the declination at a given specified See also:epoch has the same value . The information being of special use to sailors, the preparation of magnetic charts has been largely the work of See also:naval authorities—more especially of the hydrographic See also:department of the British See also:admiralty . The See also:object of the admiralty See also:world charts—four of which are reproduced here, on a reduced See also:scale, by the kind per-See also:mission of the Hydrographer—is rather to show the general features boldly than to indicate See also:minute details . Apart from the immediate necessities of the case, this is a counsel of prudence . The observations used have mostly been taken at dates considerably anterior to that to which the See also:chart is intended to apply . What the sailor wants is the declination now or for the next few years, not what it was five, ten or twenty years ago . Reliable secular change data, for reasons already indicated, are mainly obtainable from fixed observatories, and there are enormous areas outside of Europe where no such observatories exist . Again, as we shall see presently, the rate of the secular change sometimes alters greatly in the course of a comparatively few years . Thus, even when the observations themselves are thoroughly reliable, the prognostication made for a future date by even the most experienced of chart makers may be occasionally somewhat wide of the mark . Fig. i is a reduced copy of the British admiralty declination chart for the epoch 1907 . It shows the isogonals between 70° N. and 65° S. See also:latitude . Beyond the limits of this chart, the number of exact measurements of declination iswhose centre is the pole . At all points on the circle the positions of the needle will be parallel; but whereas the north pole of the magnet will point exactly towards the centre of the circle at one of the points where the straight line drawn on the ground cuts the circumference, it will at the opposite end of the See also:diameter point exactly away from the centre . The former part is clearly on the isogonal where the declination is 0°, the latter on the isogonal where it is 18o° . Isogonals will thus radiate out from the north geographical pole (and similarly of course from the See also:south geographical pole) in all directions . If we travel along an isogonal, starting from the north magnetic pole, our course will generally take us, often very circuitously, to the north geographical pole . If, for example, we select the isogonal of 1o° E., we at first travel nearly south, but then more and more westerly, then north-westerly across the north-east of See also:Asia; the direction then gets less northerly, and makes a dip to the south before finally making for the north geographical pole . It is possible, however, according to the chart, to travel See also:direct from the north magnetic to the south geographical pole, provided we select an isogonal answering to a small westerly or easterly declination (from about 19° W. to 7° E.) . Special interest attaches to the isogonals answering to declination 0° . These are termed agonic lines, but sailors often See also:call them lines of no variation, the term variation having at one time been in common use in the sense of declination . If we start from the north magnetic pole the agonic line takes us across See also:Canada, the See also:United States and South America in a fairly straight course to the south geographical pole . A curve continuous with this can be drawn from the south leduW f..t .. S..n. by p~.sl.iw e111, . Ltd . Cgputl.si...n of 3L . 3.4 fr4ty TLv pe.tron .n.,b . IhttLD T.In wd Yr nc~ Ls . See also:Leon .y.dWlf down f.. th,..L.rt ,Emory w.ikr. w . 'FIG . 1.—Isogonals, or lines of equal magnetic declination . somewhat limited, but the general nature of the phenomena is easily inferred . The geographical and the magnetic poles—where the dipping needle is vertical—are fundamental points . The north magnetic pole is situated in North America near the edge of the chart . We have no See also:reason to suppose that the magnetic pole is really a fixed point, but for our present purpose we may regard it as such . Let us draw an imaginary circle See also:round it, and let us travel round the circle in the direction, west, north, east, south, starting from a point where the north pole of a magnet (i.e. the pole which in Europe or the United States points to the north) is directed exactly towards the astronomical north . The point we start from is to the geographical south of the magnetic pole . As we go round the circle the needle keeps directed to the magnetic pole, and so points first slightly to the east of geographical north, then more and more to the east, then directly east, then to south of east, then to due south, to west of south, to west, to north-west, and finally when we get round to our See also:original position due north once more . Thus, during our course round the circle the needle will have pointed in all possible directions . In other words, isogonals answering to all possible values of the declination have their origin in the north magnetic pole . The same remark applies of course to the south magnetic pole . Now, suppose ourselves at the north geographical pole of the earth . Neglecting as before diurnal variation and similar temporary changes, and assuming no abnormal local disturbance, the compass needle at and very See also:close to this pole will occupy a fixed direction relative to the ground underneath . Let us draw on the ground through the pole a straight line parallel to the direction taken there by the compass needle, and let us carry a compass needle round a small circlegeographical to the south magnetic pole at every point of which the needle points in the geographical meridian; but here the north pole of the needle is pointing south, not north, so that this portion of curve is really an isogonal of 18o° . In continuation of this there emanates from the south magnetic pole a second isogonal of or agonic line, which traverses See also:Australia, See also:Arabia and See also:Russia, and takes us to the north geographical pole . Finally, we have an isogonal of 18o°, continuous with this second isogonal of 0° which takes us to the north magnetic pole, from which we started . Throughout the whole area included within these isogonals of 0° and 180°—excluding locally disturbed areas—the declination is westerly; outside this area the declination is in general easterly . There is, however, as shown in the chart, an isogonal of 0° enclosing an area in eastern Asia inside which the declination is westerly though small . § 7 . Fig . 2 is a reduced copy of the admiralty chart of inclination or dip for the epoch 1907 . The places where the dip has the same value See also:lie on curves called isoclinals . The dip is northerly (north pole dips) or southerly (south pole dips) according as the place is north or south of the isoclinal of 0° . At places actually on this isoclinal the dipping needle is horizontal . The isoclinal of o° is nowhere very far from the geographical See also:equator, but lies to the north of it in Asia and See also:Africa, and to the south of it in South America . As we travel north from the isoclinal of 0° along the meridian containing the magnetic pole the dipping needle's north pole dips more and more, until when we reach the magnetic pole the needle is vertical . Going still farther north, we have the dip diminishing . The northerly inclination is considerably less in Europe than in the same latitudes of North America; and correspondingly the southerly inclination is less in South America than in the same force . The total force is least in See also:equatorial regions, where values latitudes of Africa. slightly under 0.4 C.G.S. are encountered . In the See also:northern hemis- Fig . 3 is a reduced copy of the admiralty horizontal force chart phere there are two distinct See also:maxima of total force . One of these for 1907 . The curves, called isomagnelics, connect the places where so-called foci is in Canada, the other in the north-east of See also:Siberia, the P . 9, /- it/y%' •See also:im, •O j ~//, ~' / % j fib .. •'''' .4.V 55 4"; / .405 ~{~5% ~~ '260 /// .. ~jryyy ~~ ~ ~ ` .. :Xf iEl _ .._/7 1, - ' `• _ ' '' a .y •:600 =••' :., .' .. d-om . •!Y +.~.~-_ . the horizontal force has the same value; the force is expressed in C.G.S. See also:units . The horizontal force vanishes of course at the magnetic poles . The chart shows a maximum value of between 0.39 and 0.40 in an See also:oval including the south of See also:Siam and the China See also:Sea . The horizontal force is smaller in North America than in corresponding latitudes in Europe . Charts are sometimes drawn for other magnetic elements, especially vertical force (fig . 4) and total force . The isomagnetic of zero vertical force coincides necessarily with that of zero dip, and there is in general considerable resemblance between the forms of lines of equal vertical force and those of equal dip . The highest values of the vertical force occur in areas surrounding the magnetic poles, and are fully 50% larger than the largest values of the horizontalformer having the higher value of the force . There are, however, higher values of the total force than at either of these foci throughout a considerable area to the south of Australia . In the northern hemisphere the lines of equal total force—called isodynamic lines—form two sets more or less distinct, consisting of closed ovals, one set surrounding the See also:Canadian the other the Siberian See also:focus . § 8 . As already explained, magnetic charts for the world or for large areas give only a general idea of the values of the elements . If the region is undisturbed, very fairly approximate values are derivable from the charts, but when the highest accuracy is necessary the only thing to do is to observe at the precise spot . In disturbed areas local values often depart somewhat widely from what one would infer from the chart, and occasionally there are large differences 70 60 48 30 r0 6 ,20° r40 :6o° rio 0 4 9 , tic so 9, q b~ I~j // O /tY w .. ~-1f // ~~[ HiY//.~/.IH~ /.b~°:~~•.m~" ym// . - <'Uwy tZ ~1 _A'"?i : /6V0'a76IDw..do x////.197 `:~D 4kitfri 4n boy -q.-~ _zf .. • . um: 1/ 0 Amummmmilmmizoilote.m......mcii .. -3° 110r't . N) 4'7 See also:URI 16!L . •1s^ "NIL, 80° W . 6o''11,' . 46w . 60°E . 80 E. rro E. r45E . 946E . .66E . :8o 966W . 196'W . :2u° W . 1o6 W . 86W . a«L.wa fr .. . .4.:9.99 N9...1e..9 w• 1,9.1 . C...nu.ium ... 0.. wan.;..~ir See also:emery Wsikar i4 . 70" 50 See also:a4 s 40 SQ 6o between places only a few miles apart . Magnetic observatories usually publish the mean value for the year of their magnetic elements . It has been customary for many years to collect and publish these results in the annual See also:report of the See also:Kew Observatory (Observatory Department of the, See also:National See also:Physical Laboratory) . The data in Tables I. and II. are mainly derived from this source . The observatories are arranged in See also:order of latitude, and their geographical co-ordinates are given in Table II., See also:longitude being reckoned from Greenwich . Table I. gives the mean values of the declination, inclination and horizontal force for See also:January I, 1901; they are in the main arithmetic means of the mean annual values for the two years 1900 and 1901 . The mean annual secular changes given in this table are derived from a short period of years—usually 1898 to 1903—the centre of which See also:fell east all over Europe, and the rate at which it is moving seems not to vary much throughout the See also:continent . The needle is also moving to the east throughout the western parts of Asia, the north and east of Africa, and the east of North America . It is moving to the west in the west of North America, in South America, and in the south and east of Asia, including Japan, south-east Siberia, eastern China and most of See also:India . § 9 . The information in See also:figs . I, 2, 3 and 4 and in Tables I. and II. applies only to See also:recent years . Owing to secular change, recent charts differ widely from the earliest ones constructed . The first charts believed to have been constructed were those of Edmund See also:Halley the astronomer . According to L . A . Bauer,' who has made a special study of the subject, Halley issued two declination charts for the epoch 1700; one, published in 1701, was practically confined to the See also:Atlantic Ocean, whilst the second, published in 1702, contained Magnetic Elements and their Secular Change . S ,y /ma y " a! e' ~2 , . a //~ i•g O ~L~/PJ .! ~i~~1 `b / / a • . // ?Asia "j////// ~,Ej'y%y y ~a j/~~~ ////j / ~ ~/%%%/'/' q ~ )4/ • f//~ . %//.r///// ^ . . ................ /o 4014rivims,zavi,rzArif/A or ~~ i ' „` =~Jl//~// . -~~j ~~'y///~~i-v e/ar0/////'~~ gym- .h` ./ rr .. r' ' JO' - .. .~ ~~ Qi• mG lI Y eo° ` 6_._ homaahon.al p..o.i..See also:loo . . . See also:lady 4oaaaf.oiaen of Ma £dmirJy Smefy mater ay Fin . 4.—Isomagnetics, lines of equal vertical force . at the beginning of 1901 . Table II. is similar to Table I., but includes vertical force results; it is more extensive and contains more recent data . In it the number of years is specified from which the mean secular change is derived; in all cases the last year of the period employed was that to which the absolute values assigned to the element belong . The great See also:majority of the stations have declination west and inclination north; it has thus been convenient to attach the + sign to increasing westerly (or decreasing easterly) declination and to increasing northerly (or decreasing southerly) inclination . In other words, in the case of the declination + means that the north end of the needle is moving to the west, while in the case of the inclination + means that the north end (whether the dipping end or not) is moving towards the See also:nadir . In the case, however, of the vertical force + means simply numerical increase, irrespective of whether the north or the south pole dips . The unit employed in the horizontal and vertical force secular changes is Iy, i.e. o•0000I C.G.S . Even in the declination, at the very best observatories, it is hardly safe to assume that the apparent change from one year to the next is absolutely truthful to nature . This is especially the case if there has been any change of instrument or observer, or if any alteration has been made to buildings in the immediate vicinity . A change |