MAGNETISM places at which the dipping needle becomes exactly vertical, and to which the above most nearly approach, are usually styled the magnetic poles; but, as will be seen presently, they not only do not coincide with, but are at a great of the magnetic force is a maximum--a circumstance which it is necessary to keep in mind, as the term pole is also frequently applied to the points of greatest force. Like the declination, the dip is subject to secular, annual, and diurnal changes. At London it was observed by Graham, in 1720, to be 74° 42; and by Captain Kater, in 1830, to be 69° 38'. The mean of the Greenwich observations for 1865 is 68° 0'; whence it appears that the dip at London is diminishing. The annual variation change in its mean direction was sometimes as much as 5° in the course of a day. In the Philosophical Transactions for 1759 there is a paper by Canton, giving the results of observations on the daily variation during 603 days. On 574 of those days the deviations were re-distance from, the places at which the intensity gular, and nearly the same as those found by Celsius; on the remaining days they were irregular, and on those days there were displays of aurora. Canton ascribed the diurnal change to the effect of the sun's rays in heating the earth, and thereby diminishing the magnetic force; so that in the morning, when the earth is heated on the eastern side of the meridian, the magnetic force on that side is weakened, and the needle consequently declines to the west; and in the afternoon, as the earth becomes heated on the western side of the meri-is scarcely perceptible. At Toronto and Hodian, the needle begins to move in the opposite direction, and declines to the eastward of its mean place. Similarly Faraday has ascribed the diurnal variation to the heating of the atmosphere, by which the magnetism of its constituent oxygen is diminished. The amount of the diurnal change is different at different places, and at different seasons of the year. At London, for the years 1817-1819, it was found by Colonel Beaufoy to be from 4 to 5' in December and January, and above 11' in June and August. At the Royal Observatory the mean diurnal range for the year 1846 was, in summer, 15′ 14′′; in winter, 11′ 53′′; and the mean for the whole year, 13' 34". At Toronto and Hobarton the diurnal range is also greatest in the summer months and least in the winter months, and the changes take place at nearly the same hours of local time. Inclination or Dip.-The inclination has a general dependence on the latitude, but it varies considerably at different places under the same latitude. A line drawn through all those places where the dipping needle becomes horizontal (usually, though perhaps improperly, called the magnetic equator) appears to intersect the equator of the earth in four points. In the Atlantic Ocean, between Africa and America, it lies wholly to the south of the terrestrial equator, its greatest south latitude being about 25°. According to Hansteen the nodes or points intersective with the equator of the earth are on the meridians 22° E., 187° E., 120° W., and 108° W.; but these determinations must be taken as subject to considerable uncertainty, the observations being by no means accordant. The other isoclinal lines follow, with more or less irregularity, the direction of the magnetic equator until a high latitude is attained. In Boothia Felix, in latitude 70° 5' 17" N., and longitude 96° 45' 48" W., Sir James Clarke Ross found the dip to be 89° 59′, within a minute of the vertical; and the needle had there altogether lost its directive power. In the southern hemisphere, on board the Erebus, in 1841, Sir J. C. Ross observed an inclination of 88° 36′ in latitude 75° 22′ S., and longitude 161° 48′ E. (Sabine, Phil. Trans. 1843.) The barton the range of the diurnal variation of the dip is about 1'25. Intensity. The absolute intensity of the magnetic force, like the declination and dip, varies at different places, and probably at different times. It is compared at different stations by observing the number of oscillations made in a given time by a magnetic bar suspended horizontally by a fine wire or silk fibre. From the large table of results given by Hansteen, it appears that if the intensity at the magnetic equator in Peru (where it was supposed by Humboldt to be a minimum) be unity, then the intensity at Naples is 1.275, at Marseilles 1-294, at Madrid 1.394, at Paris 1.348, at London 1.370, at Christiania 1420, in Baffin's Bay (lat. 76° 45′ N., long. 58° 20′ W.) 1.705. The maximum intensity hitherto observed is 2:052, and the minimum 0.706. Both observations were made at places in the southern hemisphere, the former by Sir James Clarke Ross, in lat. 73° 47′ S., long. 171° 50′ E. (of London), where the dip was observed to be 87° 4′; and the latter by Erman, in lat. 19° 59′ S., and long. 35° 4′ W., off the eastern coast of Brazil, where the dip is 7° 55': the two extreme intensities are therefore in the ratio of 1: 2.906. In both hemispheres there are two points, or rather regions, of maximum intensity, which may be regarded as centres of separate magnetic action. The position of the principal or strongest northern maximum intensity was ascertained by Captain Lefroy, in 1842 and 1843, to be in lat. 52° 19′ N., long. 268° E. (Sabine, Phil. Trans. 1846); that of the minor maximum was determined in 1828 and 1829 by Hansteen, and Erman, in the Russian expedition, to be in the meridian of 111° 27' E. The interval between the two meridians is consequently 270° - 111° 159°, so that they are not directly opposite. Both points appear to have undergone progressive changes from west to east, though not at the same rate. In the southern hemisphere the two centres of maximum force are still nearer each other in respect of geographical longitude, and the intensity is greater than in the northern hemisphere. (Sabine, Phil. Trans. 1850.) The isodynamic lines are in general nearly parallel to each other, and also to the MAGNETISM isoclinal lines, though at some particular places the two sets of lines are nearly at right angles. The intensity does not appear to be affected by elevation above the surface. Simultaneous Perturbations.-In the volume of Hobarton Observations, 1841 to 1848, Colonel (now General) Sabine gives an interesting comparison of the simultaneous affections of the three elements at Hobarton and Toronto, on one of Gauss's term days, April 21, 1842, these two stations being specially selected for comparison as being geographically situated at nearly opposite points of the globe. The result showed that the amount of disturbance on this day was nearly the same at both stations, and that the instants of extreme disturbance are not unfrequently as nearly identical at both as the nature of the observations enables us to determine. The inference is, that many of the perturbations or particular disturbances observed must be regarded as general affections of the whole globe, manifesting themselves at stations the most remote from each other. Theory. Mr. Balfour Stewart recently delivered a valuable discourse on this subject at the Royal Institution, from which the following remarks are extracted, as they show at once the mode of investigation now pursued, and the theory now generally accepted. (Proceedings of the Royal Institution for 1863.) Gauss, who has done so much to further the science of magnetism, showed, by means of a preconcerted system of observation, that magnetic storms affected the needle at Göttingen and at other stations in Europe at precisely the same moment of absolute time; and after the establishment of the colonial observatories, it was found by General Sabine that the needle was affected in Toronto at precisely the moment when it was disturbed at Göttingen. Nor is it too much to say, with our present knowledge, that these remarkable disturbances break out at the same moment over every portion of our globe. 'Having thus shown that these phenomena are cosmical in their character, the next point of interest is their connection with the sun. This has been placed beyond doubt chiefly through the labours of General Sabine, who found at Toronto and elsewhere that magnetic disturbances obey a law of hours. Mr. Broun also showed the same thing from his observation of the needle at Makerstoun, in Scotland. It may be instructive to point out how this proof was deduced from the colonial observations; and to make the matter plain, let us refer to an imaginary case in the familiar science of meteorology. Suppose that while an observer is watching his thermometer there is a sudden influx of cold weather, and that it is wished to estimate the influence of this upon the thermometer on a given day and at a given hour on that day, what must the observer do? He must endeavour to ascertain, by the best possible means, what indication the thermometer would have afforded at that specified day and hour had there been no cold weather. Comparing this with the actual height of the mercury, and deducting the one from the other, he would clearly obtain a measure of the effect of the cold weather upon the thermometer. 'A similar course was pursued by General Sabine in discussing the colonial magnetic observations, with the object of deducing the laws of disturbances. It was first necessary to ascertain by the best possible means what position the magnet would have assumed at any particular day and hour, had there been no disturbance. Calling this the normal value, the next course was to group together, as disturbed, all those positions of the magnet which differed from the normal by more than a certain small quantity. The necessity for this separation will become evident when it is remarked that the disturbed and the regular observations have different hourly turningpoints, and obey very different laws. Thus a disentanglement was effected, which was accomplished by the employment of a separating value. The selection of this value is to some extent arbitrary, but it was shown by reference to a diagram that the disturbance law at Kew was virtually the same, whether this were deduced (in the case of the declination) from ninety-five days of principal disturbances or from all disturbed observations which differ from the normal by more than 3'-3. It was also shown from the same diagram that easterly disturbances prevail at Kew during certain hours of the day, and westerly disturbances at certain other hours, thus exhibiting a daily law, and showing that disturbances are therefore connected with our luminary. 'There is, however, a more interesting and mysterious connection than this. Professor Schwabe, of Dessau, has now for nearly forty years been watching the disc of the sun, and recording the groups of spots which have been visible, and he finds that these have a period of maximum nearly every ten years, two of these periods being the years 1848, 1859. Now it was likewise found by General Sabine, that the aggregate value of magnetic disturbances at Toronto attained a maximum in 1848, nor was he slow to remark that this was also Schwabe's period of maximum sun-spots, and it was afterwards found, by observations made at Kew, that 1859 (another of Schwabe's years) was also a year of maximum magnetic disturbance. This fact is eminently suggestive, and brings us at once into the presence of some great cosmical bond, different from gravitation, adding at the same time additional interest as well as mystery to these perplexing phenomena. 'These are the grounds on which we suspect the sun to be the agent which causes magnetic disturbances, but there is also some reason to believe that on one occasion our luminary was caught in the very act. On the 1st of September 1859, two astronomers, Messrs. Carrington and Hodgson, were independently observing the sun's disc, which exhibited at that time a MAGNETISM very large spot, when about a quarter past eleven they noticed a very bright star of light suddenly break out over the spot and move with great velocity across the sun's surface. 'On Mr. Carrington sending afterwards to Kew Observatory, at which place the position of the magnet is recorded continuously by photography, it was found that a magnetic disturbance had broken out at the very moment when this singular appearance had been observed. 'The next point to be noticed is, that magnetic storms are always accompanied by aurora and by earth currents. With regard to the latter of these phenomena, a single word of explanation may be necessary. Earth currents are currents of electricity which traverse the surface of our globe, a portion of which is caught up by the telegraphic wires, which are often thereby seriously disturbed in their communications. A table was then referred to which showed that aurora and earth currents have the same ten-yearly period as sun-spots and magnetic disturbances, so that a bond of union exists between those four phenomena. "The question next arises, What is the nature of this bond? Now, with respect to that which connects sun-spots with magnetic disturbances we can as yet form no conjecture; but we may, perhaps, venture an opinion regarding the nature of that which connects together magnetic disturbances, aurora, and earth currents. And here we may remark that this latter bond is the more definitely determined of the two, since the three phenomena which it embraces invariably occur together. 'In order to exhibit the evidence upon which this hypothesis rests, it is necessary to refer to what is done at the Kew Observatory. 'Reference was then made to a diagram in which the three curved lines photographed from the magnetometer were exhibited for September 1-2, 1859; and it was seen that about four o'clock in the early morning of September 2, the three components of the earth's magnetism at Kew were simultaneously and abruptly disturbed, and were kept at one side of their normal or undisturbed positions for many hours. During this time there were vivid aurora which extended over the greater part of the globe, and even to as low a latitude as Cuba, and strong earth currents were also observed by Mr. C. V. Walker, on the various telegraphic lines. These currents were found to change their direction every two or three minutes, going alternately from positive to negative, and back again to positive. It is therefore evident that currents varying in this manner could not have been the cause of magnetic disturbances in which the needle was kept on one side of its nominal position for many hours. But the curves of magnetic disturbance further exhibit sharp peaks and hollows, or wavelets, superimposed upon the great disturbance wave, and these wavelets change their direction every two or three minutes, in which respect they are comparable with earth currents. May not these wavelets be connected with earth currents and aurora, and may not this connection be of the following kind? A peak denotes a small but rapid change of the earth's magnetic force in one direction, and a hollow a similar change in the opposite direction Now in a Ruhmkorff's coil we have-1st, a soft iron core, with a current circulating round it; 2nd, an insulator round the current; 3rd, a secondary coil above the insulator, containing perhaps several miles of fine wire. In this arrangement we have a discharge between the terminals of the secondary coil every time contact with the primary current is made, and one of an opposite character every time this contact is broken. 'But the chief use of the primary current is to reverse the magnetism of the iron core; and could we reverse this, or even change it rapidly without a primary current, we should have the same effect, that is to say, we should have a secondary current in one direction, when the magnetism of the core was rapidly increased, and one in an opposite direction, when this was rapidly diminished. The body of our earth may be likened to the soft iron core of a Ruhmkorff's machine, in which one of the small curve-peaks already alluded to denotes a rapid change of magnetism in one direction, and a hollow a change of the opposite character. The lower strata of the atmosphere again resemble the insulator of the Ruhmkorff's machine, and the upper and rarer strata the secondary conductor; again, the crust of the earth being permeated with moisture, becomes a conductor, and may therefore also be likened to the secondary coil. Whenever, therefore, we have a curve-rise, that is to say, a sudden change of the earth's magnetism in one direction, we should have in the upper strata of the atmosphere and in the crust of the earth currents of one kind; and when we have a curve-fall or a sudden change of magnetism in the opposite direction, we should have similar currents of an opposite description. 'It need hardly be remarked, that those currents which take place in the upper strata of the atmosphere will form aurora, while those in the crust of the earth will constitute earth currents. 'Now, if this be the nature of that connection which subsists between magnetic disturbances, earth currents, and auroræ, may we not extend our enquiries, and ask, If the sun's action is able to create a terrestrial aurora, why may he not also create an aurora in his own atmosphere? It occurred independently to General Sabine, Professor Challis, and Mr. Balfour Stewart, that the red flames visible during a total eclipse may, indeed, he solar aurora. In support of this hypothesis it may be remarked that, during the late total eclipse in Spain, Mr. de la Rue, by means of the Kew photo-heliograph, proved that these red flames belong to the sun, and that they extended in one case to the distance of 70,000 miles beyond his photosphere. But, considering the gravity of the sun, we are naturally unwilling to suppose that there can be any MAGNETITE considerable amount of atmosphere at such a distance from his surface; and we are therefore induced to seek for an explanation of these red flames amongst those phenomena which require the smallest possible amount of atmosphere for their manifestation. Now the experiments of Mr. Gassiot and the observed height of the terrestrial aurora alike convince us that this meteor will answer our requirements best. And besides this, the curved appearance of these red flames, and their high actinic power, are bonds of union between these and terrestrial aurora. These prominences have since been otherwise explained. [SUN.] MAGNETOMETER dodecahedrons, also massive and in the form of sand. Magneto-electricity. Under the term ELECTRO-MAGNETISM will be found the description of certain magnetic phenomena produced by electricity. It has been demonstrated by Faraday that electric phenomena may be produced by magnetism, and to these the term magneto-electricity has been applied. Let a represent a hollow helix of copper wire covered by Wy silk, the ends bc of which are connected with a delicate galvanometer; and N S a powerful bar 'It has been remarked by General Sabine, that magnet, which can easily be thrust into and an auroral outburst in the sun may perhaps be withdrawn from the spiral or helix it will responded to simultaneously by the different then be found that every time the magnet is planets. If this be true, our whole solar sys- pushed into the helix the galvanometer is detem would seem to thrill almost like a living flected in one direction, and each time it is being under the excitement of this mysterious withdrawn it is deflected in the opposite direcforce. It has been likewise found by Mr. Gassiot, tion. On repeatedly threading the helix with that electricity cannot pass through a perfect the magnet, the deflection also takes place. vacuum, so that perhaps we have only to observe the greatest height attained by a terrestrial aurora and by a solar red flame, in order to be able to assign the limit, not only of our own atmosphere, but also of that of our luminary. 'One other point remains to be noticed in connection with magnetic disturbances, and this is, that there appear to be two separate disturbing forces, nearly opposite in character, both connected with the sun, which act simultaneously upon the magnet; the position which the latter assumes being due to the combined effect of both. This has been shown to be true by General Sabine, who has observed that the curve which exhibits the daily range of the east component of the disturbing force, is in many places very different in character from that which exhibits the same for the west component. And this difference between the two curves is of one kind at one station, and of another kind at another station. This duality of the disturbing forces may also be observed directly in the Kew disturbance-curves. Now, as the deflection of the galvanometer can only be produced by the motion of electricity in the helix, it is obvious that an electric current is produced each time that the magnet moves through it; hence as, on the one hand, electricity in motion produces magnetism, so, here, magnetism in motion produces electricity. By causing the pole of a powerful magnet to revolve before a coil of wire, or, what amounts to the same thing, if the coil be made to revolve opposite to the pole of a magnet, an electric current will be established in the coil, which may be made sensible by sparks, shocks, and chemical effects; and by employing a combination of such magnets and coils a current sufficiently powerful for the electric light may be obtained. In magneto-electricity the source of power is the mechanical force used to rotate the magnet or coil. This force may be supplied by muscular action, as in Wheatstone's magnetoelectric telegraph instruments; or it may be derived from the steam engine, as in Holmes's and Grammes's magneto-electric light. The attention of foreign men of science has Magnetometer. An instrument for meabeen much directed to the problem of terrestrial suring the intensity of terrestrial magnetism. magnetism, and five sets of magnetographs, The three elements sought to be deduced from similar to those in operation at the Kew Ob- magnetic observations are, the declination, the servatory, have been already procured by fo- inclination or dip, and the absolute intensity, reign governments. These, however, will be together with the variations to which they are placed in the northern hemisphere, and it is subject; and each of these elements requires to be desired that some of our colonies in the for its determination a peculiar apparatus. southern hemisphere may come forward in order When adapted to the purpose of determining that by the next epoch of maximum disturbance the declination, the magnetometer is called a (1869) there may be such a network of mag- declination magnetometer; when for the horinetic observatories as may enable us to obtain zontal and vertical force, they become horizontal the solution of this interesting and important and vertical force magnetometers. From the problem.' experience obtained in the British colonial obMagnetite. Magnetic Iron-ore, or Oxy-servatories, it appears that these instruments, at dulated Iron. One of the richest and most least in the form recommended in the Instrucimportant of the ores of iron, and that from tions drawn up by the Committee of the Royal which the finest kinds of steel are made. It is a widely diffused mineral, and is found abundantly in many localities, especially in Lapland, Norway, Sweden, and Canada, occurring crystallised in iron-black octahedrons and VOL. II. 433 Society, are liable to certain sources of irregularity; one depending on the liability of the magnetised bar to lose a portion of its magnetism, and others on causes not perfectly understood. (Sabine, Phil. Trans. 1850.) At Green FF MAGNETOMOTOR MAID OF HONOUR wich, Kew, and other important magnetical Magnoliaceae (Magnolia, one of the geobservatories, the magnetometers are selfrecording, a mirror attached to them reflecting a beam of light on to photographic paper kept in motion by proper apparatus. For particulars of the construction of these instruments, see Becquerel, Traité de l'Electricité et du Magnetisme, vol. vii., and Daguin, Traité de Physique, iii. 78. Magnetomotor. A term applied to a voltaic series of two or more large plates, which, producing a great quantity of electricity of low tension, is well adapted to the exhibition of electro-magnetic phenomena. Magnifying Power. As the illuminating power of a telescope or microscope depends upon the aperture of the object-glass, so does the magnifying power depend upon the focal length and the depth or power of the eyepiece. In the telescope this power is generally limited by the illuminating power, as each increase in the size of the image spreads the light over a larger area, until the image is too dim to be observed. In the microscope, as the light can be increased to almost any extent, the magnifying power is proportioned only to the perfection of the object-glass. nera). A natural order of Exogenous plants of the Ranal alliance, consisting of trees or shrubs of great beauty, usually with evergreen leaves, and large fragrant flowers. They inhabit the temperate parts of America and Asia, as well as the tropics, and are universal objects of cultivation. The bark of the Tulip-tree, Liriodendron Tulipifera, and of some of the true Magnolias, has the reputation of being a good febrifuge. Drimys Winteri yields Winter's Bark. Magpie. A common species of the Crow tribe, Corvus Pica of Linnæus; now the type of a distinct genus, Pica caudata. They continue in pairs throughout the year, and prey on a variety of food, chiefly animal, as the young of hares, rabbits, and feathered game, young poultry, eggs, carrion, and insects; lastly, fruit and grain. Maguey. The American Aloe, Agave americana. Mahabharata. The name of one of the great Indian epic poems, the chief subject of which is a long civil war between two dynasties of ancient India, the Kurus and Pandus. This poem, which embraces the whole circle of Indian mythology, has been recast by later editors, evidently Brahmans, who have in great part changed its epic into a didactic character. This collection of poems is more recent than the Veda, for the war, which is its principal subject, is not known in the latter. (Max Müller, History of Sanscrit Literature, pp. 42-48 &c.) Many episodes from the Mahabharata have been ably translated by some of our most celebrated Orientalists; and parts of the original have been published at different periods in Germany. [RAMAYANA.] Magnitude (Lat. magnitudo). Size, extent, quantity. This term was originally employed to designate the space occupied by any figure; or, in other words, it was applied to objects strictly termed geometrical, and of three dimensions-length, breadth, and thickness: then it was extended to designate the quantity of any one of these, and also of angular space, or the inclination of two lines to one another; or, again, the compound idea of a solid angle formed by any number of planes meeting in a point. The amount of any one of these, taken Mahadeva (Gr. μéyas béos, great god). In in reference to some standard of the same kind the Mythology of the Hindus, the name of a of quantity as that spoken of, was called its deity who shares the attributes of Siva in the magnitude. The term was gradually enlarged later Indian Trimurtti, or Trinity. These attriin its signification, so as to apply to every kind butes vary greatly, Mahadeva being regarded of quantity that admits of exhibition or men- as a generator as well as a destroyer. In the suration, or of which greater or less can be pre- earlier Vedic writings Mahadeva, like Rudra, is dicated; and in this sense it was used by Euclid. the name of a god who is described under chaMagnitude, Apparent. The angular space racters as various as those which are assumed (plane or solid) under which a body appears by Heracles and Phoebus in Hellenic Mythology. when viewed from some distant point. The (Muir, Sanscrit Texts, part iv. ch. iii. sect. 7.) term is chiefly used in speaking of the celestial Maharanga (its Nepalese name). bodies, and is then employed to express the genus of Boraginacee, of which a Nepal speplane angle subtended by the diameter of the cies, M. Emodi, yields thick fleshy deep purple visual disc of the body. It is also used in roots, which impart a brilliant red to oils. many branches of optical science, but always They are the Rutton roots of the Indian with the same general meaning. bazaars, and oil coloured by them is used for staining wood of a mahogany colour. Magnolia (after Pierre Magnol, professor of medicine at Montpelier). A fine genus of trees or shrubs, some of them evergreen, found in North America, Northern India, and China. Many of them are cultivated for their beautiful tulip-like flowers, often of large size, and deliciously fragrant. One evergreen species, M. grandiflora, adds to shining laurel-like leaves large white aromatic flowers of great beauty. M. glauca is called in America the Swamp Sassafras, and has qualities resembling those of the true Sassafras. A Mahogany. The timber of the tree known as Swietenia Mahagoni. This is the true or Spanish mahogany. Indian mahogany comes from Cedrela Toona; and African from Khaya senegalensis. Mahometanism. [MOHAMMEDANISM.] Maia. In Greek Mythology-1. A daughter of Atlas and Pleione, and mother of Hermes. 2. A Roman divinity, also called Majesta. Maid of Honour. An attendant of high rank on the person of the queen. [HOUSEHOLD.] |