Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/173

Rh TERRESTRIAL MAGNETISM.] METEOROLOGY 163 compare the instrumental readings on days when the tempera ture is high with the readings on days when the temperature is low. 24. By differential instruments the components of a force affect ing the magnet are determined in three directions at right angles to each other. It does not, however, follow that this force is entirely due to changes in the magnetism of the earth. We know that certain forces connected with the sun affect the earth s mag netism, and on certain occasions at least these forces manifest them selves as currents in the upper regions of the atmosphere and in the crust of the earth. Now such currents will have a direct effect upon the needle as well as an indirect effect through the changes which they may produce in the magnetism of the earth. The total influence on the needle will therefore be made up of these two ele ments, the one denoting the direct influence on the needle of the disturbing force, and the other the indirect influence through the change produced in the earth s magnetism. No attempt has yet been made to separate the action of these two elements. 25. Self-recording instruments after the Kew pattern have been supplied to observatories at the following places : Batavia. Mauritius. Coimbra (Portugal). Kolaba (Bombay). Lisbon. Vienna. St Petersburg. Zi Ka Wei (China). Florence. San Fernando (Spain). Stonyhurst. Potsdam. Utrecht (declination only). Brussels. Melbourne. Nice. There are also self-recording magnetographs of other patterns at Toronto, Montsouris (Paris), Greenwich, Wilhelmshaven (?), Cape Horn, and Havana (?). We understand that Professor W. G. Adams is at present engaged in making a comparison of simultaneous curves from vari ous stations of these lists.* MAGNETIC POLES OF THE EARTH SECULAR VARIATION. 26. Magnetic Poles of the Earth. In the article MAGNETISM it has been shown that Dr Gilbert of Colchester had at a very early period grasped the important truth that the earth is a magnet, a truth which was afterwards mathematically demonstrated by Gauss. It was reserved for Halley, the contemporary of Newton, to show that the earth must be regarded as having two poles in the northern and two poles also in the southern hemisphere, so that, unlike ordinary magnets, its magnetic system has four poles altogether. Before proceeding further it will be desirable to state what it was that Halley actually did and what are the conclusions to be derived from his investigations. It has been remarked by Professor Stokes that, while in an ordinary bar magnet we may practically regard the pole as having a physical reality and as being the cause of well- known attractions and repulsions, we are not entitled a priori to assume that a point of maximum force in a large spherical magnet like the earth must necessarily be the seat of attractions and repul sions after the same manner as the pole of an ordinary bar magnet. It is to be determined by observation to what extent the earth actually preserves an analogy to an ordinary magnet. Now Halley s conclusions were derived from the pointing of the declination needle, since in his day there were no observations possible on total magnetic force. He argued that there are two points or poles in the northern hemisphere to which the needle appears to be attracted, one in the upper region of America and one above Siberia. So far this conclusion is hardly anything more than a formal one derived from the grouping together of observations. He asserted that these would be as they are known to be if we imagine two such poles or foci of force each exercising a causal influence on the magnetic needle. And the justification of Halley s way of regarding the earth is found in the fact that when force observations came to be made two such foci of force were actually found to exist. We do not, however, mean to imply that these foci have causal properties exactly similar to the poles of a bar magnet, for this is not the case. In order to exhibit the process of reasoning which led Halley to his conclusion, let us first imagine that the earth has only a single pole or force-focus in the northern hemisphere, and that this is coincident with its geographical pole ; then, assuming that this pole has a causative influence on the needle s declination, we should expect all needles to point everywhere due north. If, however, this pole be not coincident with the north pole of the earth, let us draw a meridian circle passing through the magnetic pole and complete it round the earth so as to divide the earth into two halves. At all 1 We are indebted to Mr Gordon and to his publishers Messrs Sampson Low & Co., who have obtained them for us for the sketches of the instruments for absolute determinations, with the exception of that of Rater s compass, for which we are indebted to Mr J. J. Hicks. For the sketch of the self-recording mag netographs together and in detail we are indebted to the Kew committee and to Mr G. M. Whipple, director of the Kew Observatory. points in this meridian circle the needle might be expected to point due north, while in the one half of the earth so divided it should point to the east and in the other half to the west of true north. In the next place let us imagine that the earth has two north mag netic poles or foci of equal strength, both being at the same latitude, while their difference in longitude is 180, and let us draw a com plete circle of meridian passing through these poles (fig. 28). Let us start from a point in this circle under one of these poles and pursue our journey eastwards along a circle of latitude. At first the needle will point due north. As we move east wards the needle will point west wards to the pole we are leaving until we come to a region half-way between the two poles, where it will be equally solicited by each, and will therefore again point due north. Let us call the space we have travel led over since we set out A. As we proceed the needle will now be under the predominant influence of the Fig. 28. second pole to our right, and will therefore point to the east until we arrive at the meridian under the second pole. This second space which we have travelled over let us call B. As we proceed we pass through a space C where the needle again points to the west until being once more equally influenced by the two poles it will point due north. After this we pass through a space D of easterly variation until we arrive once more at the point from which we started. Thus there are now four spaces instead of two, and these are shown in fig. 28, where the centre of the circle represents the north geographical pole of the earth, and its circumference the equator. If pole 2 be inferior in power to pole 1 the spaces B and C will be smaller in size than A and D. 27. This last is an arrangement of things that agrees very well with the results of observation, if we add that the two poles are not precisely 180 removed from one another in longitude. Fig. 30 2 represents lines of equal magnetic variation in 1882. There are two lines extending throughout both hemispheres at all points of which there is no variation, and also an oval-shaped district in the northern hemisphere throughout all points in the circumference of which we have no variation. These facts are inconsistent with the hypothesis of a single pole, but they are quite consistent with that of two poles or foci of force, one in northern America and the other in northern Asia, the former being stronger than the latter. In order to see this let us take our stand at the great line of no varia tion which passes through North America and travel eastwards. We are just south of the American pole or focus, while the Asiatic pole or focus is nearly 180 off, and hence the needle points due north. As we proceed eastwards we leave the American or strongest pole to the westward of us, and hence we have a region of west variation which we have agreed to call A. As we begin to approach the eastern side of Europe we get nearer the Asiatic pole or focus, and at length the line of no variation is reached, the tendency of the American pole to pull the needle to the west being balanced by the tendency of the Asiatic pole to pull it to the east. After this, easterly variation predominates throughout a region B until at length we come to a point in the western boundary of the oval where we may imagine ourselves to be directly south of the Asiatic pole, while the American pole is nearly 160 distant ; once more the needle points due north. As we still travel eastwards we leave the Asiatic pole, which is now the predominant one, to our left, and hence we have here a region C of westerly declination. At length we come to the eastern boundary of the oval, where the tendency of the Asiatic pole to pull the needle to the west is balanced by the tendency of the American or stronger pole (acting now towards the right) to pull it to the east, so that we have once more a point of no variation. After this the American pole predominates, and we have a large region D of easterly variation until we travel round once more to the point from which we started. 28. This train of argument receives, as we have already men tioned, corroboration from the fact that in the map of total force we perceive two foci of maximum force, one in northern America and the other in northern Asia, that in America being the strongest. This evidence was not, however, in existence at the time of Halley, and his hypothesis of two poles does the greater credit to his sagacity, inasmuch as he had to deduce it from a comparatively small number of observations of declination and dip, those of force being altogether wanting. 29. We have hitherto spoken of two poles or, more properly, foci of maximum force, the positions of which are of course best pointed out in fig. 29 ; but we have seen that the existence of such - We are indebted for the admirable charts given in figs. 29-32 to the kindness of the hydrographer, Captain Sir Frederick Evans, who, in order to save time, allowed us to make use of the information he had embodied even before it was officially published, and who likewise placed his plates at our disposal.