B TERRESTRIAL MAGNETISM the earth is a great magnet, and that the need] assumes a N. and 3. direction because it is at by the dissimilar and repelled by th tilar poles of the terrestrial sphere. H tte<lthishvp<>th(-is i iy magnetizing ana globes of steel ; but this illustration, though i served in a general way to represent the phe nomena, is not strictly correct. In the firs place, the magnetism of the earth is not sym metrical like that of a steel magnet, but is to I Arable degree irregular; and secondly, i is not permanent, but subject within certaii limits to almost continual changes both in di n and intensity. Indeed, the magnetu ly ever absolutely stationary fnun one moment to another, but is constantly exhibiting minute variations. If the earth is a magnet, the free needle at any place should as same a definite direction ; but it does not fol- low from the hypothesis that this direction mu-t be the true north and south, since the magnetic poles of the earth do not necessarily coincide with its geographical poles. If the two pole-* he in the same meridian with a given 'he needle will at that place point to the true north ; but if the magnetic pole lie either W. or E. of the meridian of a given place, the S. end of the needle will deviate either E. or W. of the true north, and the phenomenon of the declination or variation of the compass will be exhibited. That the needle does not point to the true north had long been known, and it was observed by Columbus in his first voyage of discovery that the direction of the i- not the same for all portions of the 4arth. Thousands of observations have since been made to obtain the data for constructing charts to represent for the use of the mariner the declination in various parts of the earth, if we assume that the earth is a great magnet, it will follow that in passing from the magnetic equator, the needle which is accu- rately balanced, so as to settle horizontally at the former pla<-e. will incline or dip as we ad- vance to either pole. That this is really the HIM discovered by Robert Norman in Furthermore, if the earth is a magnet should expert that the magnetic intensity the strength of the action would not be the wme at all po.i.ts of its surface, and this infer- isiOsobeen found to be true. Bycount- thc vibrations of a delicate dipping needle we find that the strength of the magnetism of he ...creases .as we go from the equator ward th, pole. The magnetic intensity, how- ever, exhibited by observations of this kind not indicate as rapid an increase of force approach the magnetic pole as might be expected from such a distribution of magnet- ism as would result from a magnetized sphere ron. In conformity with the three majr- '"nt* we have mention,-,! nanidv the variation, the dip, and the intcnsi'tv, it !^' tnagnetic condition of Jl!! "V" ! irnob y ^ree systems of lines supposed to be drawn on the surface of the globe. These are as follows: 1, the line drawn through all places where the needle points to the true north or south, to 6 W., to 6 E., 10 W. and 10 E., and so on, called the isogonic lines, or lines of equal variation or de- clination ; 2, lines nearly at right angles to the former, drawn through all places exhibiting the same angle of dip of the needle, called iso- clinal lines; and 3, a system of lines joining all places having the same magnetic intensity, and consequently known by the name of iso- dynamic lines. It is a problem of much prac- tical importance in regard to the art of navi- gation, as well as to the study of the phenome- na of terrestrial magnetism, that these three systems of lines should be accurately deter- mined ; and accordingly expeditions have been fitted out by different nations almost expressly for this purpose. All the observations, how- ever, which have been made in regard to them, indicate the fact that they are not permanent, but are constantly undergoing a change, of which the law is exceedingly complex. Hal- ley's chart of declination for 1700 is very dif- ferent from that of Barlow for 1833 ; and Han- steen's dip chart for 1780 does not represent the isoclinal lines of the present day. The ^reat practical object then of investigation in
- his branch of science is to discover the law of
- hese changes, in order that, the position and
! orm of these lines being determined for a pven epoch, they may be calculated for any future time. The phenomena were first refer- red to a very small magnet at the centre of the earth, the direction of which is subject to Regular changes. Tobias Mayer, instead of supposing a magnet to be placed at the centre of the earth, conceived one to be situated at ibout the seventh part of the earth's radius rom the centre, and from this hypothesis he was enabled to calculate the variation and dip n places not far distant from those in which hese quantities had been determined by actual observation. Hansteen of Norway, who col- scted an immense number of observations, en- .eavored to represent the phenomena by the lypothesis of two small eccentric magnets of nequal strength placed at the centre of the arth, giving rise to four magnetic poles, two n each hemisphere. In order to represent the anations of the needle, the poles of each of hese two magnets were supposed to perform revolution around an intermediate line, with fferent velocities. Gauss of Gottingen, how- rer, made the first rigid investigation of the roblem m accordance with a definite plan. ' founded his research oil the assumption hat the terrestrial magnetic force, or that vnicn is exerted on a needle freely suspended y its centre of gravity, is the resultant action the magnetized particles of the earth's ass. According to this assumption, the so v- rning power which affects the needle is due o the magnetism of the eartli itself, while the brent perturbations to which the needle is subjected are the results of extraneous forces
