Page:The American Cyclopædia (1879) Volume XI.djvu/17

TERRESTRIAL MAGNETISM 9 To give clearness of perception, he represents magnetization as consisting in the separation of two magnetic fluids, giving magnetic polari- ty to each particle, or in other words in a re- pulsive and attractive force acting inversely as the square of the distance. No change would be produced in the result by adopting the hy- Eothesis Of Ampere, in which magnetism is eld to consist of constant magnetic currents ; nor would there be any difference if terrestrial magnetism were ascribed to a mixed origin, as consisting partly of actual electrical currents and partly of permanently magnetized masses. Starting from these assumptions, Gauss obtain- ed a general mathematical expression for the action of the whole globe on a magnetic needle, however irregular might be the distribution of the magnetism of the former. In other words, he obtained an expression by which, if the dis- tribution of the magnetism of the earth were known, and the intensity of its action ascer- tained with reference to a unit of distance and intensity, the position of the needle and the magnetic force by which it was acted upon at any point could be determined ; and con- versely, if the action of the earth on the needle were known for a large number of places on the surface of the earth, the distribution of the magnetism might be considered the unknown quantity, and might be approximately found from the data thus afforded by observation. In this way Gauss was enabled to give a meth- od of constructing general charts to represent in every part of the earth the magnetic declina- tion, inclination, and isodynamic lines, the in- tensity and direction of the magnetic force be- ing known at a given number of places. The data necessary for improved charts of this kind have been furnished by the magnetic surveys made in various parts of the world in recent times, at the suggestion and principally under the direction of the British association. By repeating the construction of such charts for different epochs, the secular changes in dif- ferent parts of the earth will become known ; and it is hoped that, in due time, if the sys- tem of magnetic observations which has been established should be continued, the law of the changes will ultimately be fully ascertained. The investigations of Gauss have shown that the hypothesis of two movable magnets at the centre of the earth does not explain the phe- nomena of terrestrial magnetism. He defines a magnetic pole to be the place at which the needle points directly downward, or at which the dip is 90. Indeed, he has pointed out the very obvious fact, that if there be two such points in the northern hemisphere, then there must be somewhere between the two a third point at which the needle would also assume the vertical position. Gauss, how- ever, arrives at the remarkable conclusion that the place of greatest magnetic intensity does not coincide with that which is usually de- nominated the pole ; and it would appear that there may be a diffused space in the northern hemisphere around which the isodynamic lines may be drawn, representing apparently at least two centres of greater magnetic attraction. These phenomena are best represented by the hypothesis of magnetism due to currents of electricity in the earth, but as yet no definite hypothesis has been advanced as to the nature of such currents. It is true, they have been referred to thermo-electricity ; but how the varying heat of the sun or the high tempera- ture of the interior can give rise to currents constantly circulating round the earth, of such intensity and such flexures as would account for the observed direction and intensity of ter- restrial magnetism, has not yet even approxi- mately been made out. What we have said in regard to the magnetism of the earth princi- pally relates to its state at a particular time. We shall now briefly give an account of the discoveries which have been made in regard to the changes to which terrestrial magnetism is subject; and for the data from which these have been deduced science is indebted to the several magnetic observatories established in different parts of the earth. These are fur- nished with improved instruments, which in their present perfect state constantly record, by means of photography, the minutest changes in intensity and direction of the magnetic force. The magnetic perturbations were at first sup- posed to consist of two classes, namely, peri- odical and fitful. Many perturbations, how- ever, which had been regarded as fitful are now known to recur at regular periods, and are therefore not properly designated by this term. The changes of terrestrial magnetism are of three classes. The first consists in a movement of the magnetic poles, around the true poles of the earth, from E. to W. in both hemispheres. This motion is inferred from the secular changes which have been found to affect the position of the magnetic lines, as .well as from the secular changes in the posi- tion of the magnetic needle at any given sta- tion. The magnetic lines at any given epoch present great irregularity of shape, because very slight differences of magnetic declination, due to local peculiarities, may largely affect the position of the magnetic lines. But when the changes of declination at any given station are considered, they are found to correspond, at least during the period within which sys- tematic observations have been made, to an oscillation such as would result from the mo- tion of the magnetic poles around the true poles of the earth in a period of between six and seven centuries. Thus in 1576 the decli- nation needle in London pointed 11 15' E. ; in 1657 or thereabouts the needle pointed due K ; in 1760 it pointed W. by 19 30'. The westerly declination attained its maximum in 1819, when it amounted to 24f. Since then the needle has been slowly travelling east- ward, the present annual rate of decrease be- ing more than 8'. The mean westerly decli- nation for the year 1873 was 19 30'. Again,