no DESIGN IN NATURE 



PLATE LVI (continued) 



field and pass into the substance of the magnetised steel bar ; a place of no magnetic force making its appearance just above s. 

 Beyond the range of the sink s, the lines of force bend round in wide curves, passing through the undisturbed side of the field and 

 completely encircling the conductor " (Ebert, p. 208). 



This picture shows how concentric lines of force can be directed and converted into spiral lines by the presence of a foreign body. 

 Spiral arrangements occasionally occur in crystals, and are very common in plants and animals. Spiral structures and spiral move- 

 ments are everywhere present in the inorganic and organic kingdoms, as witness tiie whirlpool, sand-storm, cyclone, and the movements 

 in walking, swimming, and flying. 



Fig. 10. — Lines of force figure produced by a conductor and the north pole of a bar-magnet acting on iron-filings. In this case the 

 concentric lines of force investing the conductor (c) become mixed up with a certain number of the lines of force proceeding from the 

 north pole (») of the bar-magnet. At c there are concentric lines of force, and at n radiating lines of force. In the interspace between 

 c and n, the two systems of lines become mixed and blend together. In the lower part of the picture there is a space nearly free from 

 lines of force. The current passes in a downward direction clock-wise round c. This figure shows a transition as between radiating 

 and concentric lines of force which results in a certain degree of spirality. 



Fig. 11. — Lines of force figure produced by the action of two current-conductors (placed ividely apart) on iron-filings. When the 

 conductors are separated by considerable intervals they do not disturb each other's magnetic fields. Thus in cross sections the lines of 

 force investing each conductor are disposed in concentric circular lines ; each system of lines being comjjlete in itself, and not being 

 influenced or modified by the other system. It is otherwise when the conductors are placed near each other, as seen at Fig. 12, which 

 see. This figure resembles the arrangement found in cross sections of the vascular bundles of plants and the blood-vessels, &c., of 

 animals. 



Fig. 12. — Lines of force figure produced by the action of two current-conductors (ptocedMecw each other) on iron-filings. In this case 

 each of the two axial fields extends into the range of the other, with the result that the two fields -are disturbed and considerably 

 modified. Provided the currents of the two conductors flow downA\'ards their lines of force run clock-wise around Cj and Cj. Between 

 the conductors their annular lines of force run in contrary directions, and tend to cro.ss each other figure-of-8 fashion. Exactly in the 

 middle, between c^^ and c^, there is a point where the two halves of the figure-of-8 curves meet. Within the two halves the lines of 

 force are in the form of rounded ovals, with elongated ends turned towards each other. Outside the two sets of ovals with the elongated 

 ends the lines of force pursue a common, elongated, oval course. 



This figure is important, as showing that figures-of-8 may be produced from rotatory movements in opposite directions, and by the 

 juxtaposition of the current conductors. Figure-of-8 structures and movements are very common in animals, especially in the hollow 

 viscera and organs of locomotion. 



Fig. 13.— Lines of force figure produced by a conductor and a bar-magnet acting on iron-filings. The lines of force of the current 

 produce a concentric system of rings seen at the cross section of the conductor (c). At the side where the magnet is, with its source n 

 and sink s, the two fields are superposed. The current flows from below upwards, through the plane of the figure, the lines of force, 

 viewed from above, encircling the conductor-currents counter-clock-wise, as may be seen from their behaviour on encountering the poles 

 (», s) of the magnet. If the magnet is free to rotate about its middle point it is deflected. This figure shows conclusively how con- 

 ductors and magnets mutually influence each other. 



Fig. 14.—" Shows a lines of force diagram for a ring-shaped coil, in a plane passing through the axis of symmetry. .S";), Sp are the 

 places where the coil intersects the plane of the diagram, on either side of which it projects semicircularly. The lines of force sur- 

 rounding all the separate turns follow approximately the same course, and unite to form continuous curves, those which most closely 

 embrace the cross section of the coil being nearly concentric with it. On the other hand the more remote lines, owing to the pressure 

 of the inner ones, are bowed out excentiically into widely spreading curves. In the neighbourhood of the axis of the coil the field is 

 approximately uniform " (Ebert, pp. 241-2). 



Fig. 15.— Lines of force figure produced by two oppositely-directed parallel currents and their electrodvnamic action on one 

 another (repulsion). ,\, c.2, Cross sections of the two conductors. Provided with current along c^ from above downward through the 

 plane of the figure its lines of force follow the hands of the clock. The current along c, in this case flows from below upwards, and in 

 a direction opposed to the hands of the clock. In the mesial line between the conductors (c,, c^) the lines of force which run side by 

 side have the same direction Their tensions and pressures are added togetlier. The annular'rings of force because of mutual pressure 

 are crowded together m the direction of the mesial line, and bulged out in a contrary direction, and cease to be strictly concentric, as 

 Jiappens m |ig. 6, Plate Iv. Ihe pressure across the lines of force tends to drive the two current conductors apart, as indicated by 

 the darts. In other words the current conductors repel each other. 



Figs. 16 and 17.— Show the dispositionof the lines of force in para-magnetic and dia-magnetio bodies. Thus in iron (Fe), which is a 

 para-magnetic substance, the lines of force from the north pole («) of a magnet converge before the iron is reached, and diverge after 

 passing through It The ends of the iron aciiig the north and south poles become a source and a sink. In bismuth (Bi), whkh is a 

 dia-magnetic sub-stance the hues of force behave quite differently : they diverge on reaching the bismuth and converge after having 

 passed It. They pass tor the most part through the surrounding medium. In the case of the iron there is axial or polar setting : in 

 the case of bismuth equatorial setting. Different substances are difl^erently affected by magnetism, and the substances in turn 

 react upon the currents : the lines of force m the case of para-magnetic bodies are made to converge and diverge: whereas in 

 flT',Ti^ Ji"' f T '''■' I to diverge and converge. These opposite effects are of the utmost consequence in the distribution of 



nn?l nlri !n\ Ttl ™°^'^.™™*^ "^ morganic and organic matter. They prefigure the opening or expanding centrifugal structures 

 and movements, and the closing or contracting centripetal structures and movements, everywherlobservable infants anS animals. 



The iron-fiUng pictures afford indications of the direction and magnitude of the magnetic force. 



There are in the iron-fihng pictures what are virtually outgoing and returning hnes of force. These are due 

 to opposite properties at different points of the surface of the magnet, whereby the influences which proceed from 

 certain regions of the magnet re-enter it at other regions. These regions are designated polar regions or poles and 

 are similar to the poles of the earth, " from one of which magnetic Hnes of force proceed outwards into space, while 

 at the other the Hnes pass back into the interior of the earth." 



It is convenient to speak of the points from which the Hnes of force issue and re-enter the magnet in greatest 



