114 DESIGN IN NATURE 



360 degrees or (for miners) into twenty-four hours or (for seamen) into thirty-two points. The points corresponding 

 to north, south, east, and west are generally marked more conspicuously than the rest. The instrument is enclosed 

 in a case, and is furnished with a contrivance, called an arrestment, which serves to lift the needle off the point when 

 not in use. 



" To the Itahan, Giovanni Gioja, is due the device of attaching the needle to a divided circle, or a disc marked with 

 the thirty-two points, thus producing the compass so indispensable at sea. The needle (or system of needles), with 

 the disc attached, turns upon a fixed point within a case, which is closed by a glass plate. A fixed mark within 

 the case gives the direction of the ship's head. Since the graduated disc is always maintained in the proper orienta- 

 tion by the needle attached to it, the steersman can read off immediately on the graduations the direction of 

 the ship's head, that is, the direction in which he is steering. The compass usually hangs in a system of movable 

 rings (Cardani's suspension), and its graduated disc floats upon alcohol, so as to be as little influenced as possible by 

 the oscillations of the ship." 



The earth itself is a great magnet, with north and south poles, in which the Unes of force are aggregated. It 

 has also its magnetic field and indifferent zone. 



" The intensity of the earth's field is least in the neighbourhood of the equator, and becomes greater towards 

 the poles, though not according to any regular law. The places where the magnetic force is a maximum are not 

 situated at the poles, but at some distance from them. The earth is thus seen to be an irregularly magnetised body. 

 The variations of intensity or ' total force ' may be indicated by means of curves such that for all points on any 

 one curve the ' total force ' is the same. These are called isodynamic lines. . . We must picture the lines of force 

 as emerging with considerable density in Victoria, South Australia. At greater distances from the south pole 

 (magnetic north-seeking pole), lines of force also emerge from the earth, but less thickly, and with a component 

 of direction towards the north. Still further northwards we come to the indifferent zone of the terrestrial magnet, 

 which is a somewhat broad band running equatorially round the earth. After this there are regions where the 

 lines of force from without re-enter the earth, somewhat few at first, and nearly horizontal in direction, but always 

 denser and more nearly vertical as we approach the north magnetic pole, which has the polarity we have called 

 south-seeking. Finally, when we reach this point, in the North American Archipelago, the hnes of force have some- 

 thing hke their greatest density of distribution, and run vertically downwards into the body of the earth " 

 (Plate Iv., Fig. 5). 



The magnetic field of the earth comprises all the space surrounding our planet, so far as we can detect in it 

 anjr magnetic influence. The portion of the field best known to us is the earth's crust and its atmosphere. The 

 earth affects neighbouring heavenly bodies and ordinary magnets, and is affected by them. There is reciprocity 

 as between the heavenly bodies and our planet. Matter and force are continually acting and reacting on each other 

 directly or indirectly at near or remote distances. The magnetic field of the earth is not constant, but is subject 

 to periodic variations. It is convulsed by sudden changes of condition, and is slowly and progressively altering 

 its distribution. In the variations of the field we can recognise the influence of astronomical events, so that 

 there would seem to be magnetic hnes of force connecting our planet with other worlds. The knowledge of this 

 fact gives to magnetic forces a universal interest and significance. 



" Very closely connected with the earth's system of magnetic hnes of force are the luminous phenomena which 

 at certain times produce an illumination of the upper layers of the atmosphere in both polar regions, often on a very 

 magnificent scale. They are a direct visible expression of the magnetic condition of the earth, and are so especially 

 related to the variations of this condition that they were justly called by Humboldt ' magnetic tempests ' (magnetische 

 XJngewUter). When a magnetic storm passes over the earth, and the magnetic needles are set trembling, the Ughts 

 at the northern and southern poles, at no time quite extinguished, become especially brilliant. ... The best known 

 are the ' northern lights ' or • southern hghts ' {amora borealis or australis), and consist of luminous streamers which 

 flicker unsteadily to and fro, passing downwards from the higher and more rarefied regions of the atmosphere, and 

 vice versd." i 



It is not necessary to pursue the subject of inorganic and organic electricity further ; suffice it to say that, as 

 the circle of the sciences widens, it becomes more and more apparent that it is not possible absolutely to separate 

 the morganic and organic Idngdoms either as regards their matter or their force. They merge and blend at 

 innumerable points, and practically the same laws control both. The organic kingdom is indebted for its substance 

 and not a little of its force to the inorganic, and virtually the same constructions and movements prevail in both. 

 These for the most part consist of radiating, concentric, curve, and spiral arrangements and movements seen in 

 plants and animals and parts thereof. 



1 The above quotations are from Professof H. Ebert's recent work, " .Magnetic Fields of Force," translated by C. V Bnrtmi n <5^ 

 London and New York, 1897. > n « ^ >. muton, U.bc. 



