io8 DESIGN IN NATURE 



PLATE LV (continued) 



embraces the conductor in an opposite direction. Two systems of opposite concentric rings are produced. Between the points "^i ^-'^'^ <^2 

 lies the needle n, .s, its position being indicated by the disturbance of the lines of force. Two bundles of lines of force are attachea to 

 the needle, due to the current passing in opposite directions. These meet in the middle of the magnet and form an mdiflerent zone. 

 The tension along the lines of force tends to deflect the needle out of the plane of the current, causing the north pole (n) to move 

 downwards and the south pole (s) upwards. , 



This figure shows that lines of force are influenced and modified by the presence of foreign bodies. Bodies act and react on eacu 

 other, and so of movements. c i * .q 



The figure illustrates the spiral, figure-of-8, and circular arrangements frequently met with in the tissues and organs of plants and 

 animals, and in the movements of walking, swimming, and flying. 



PLATE LVI 



Fig. 1.— Lines of force figure produced by the action on iron-filings of two magnets with their north poles turned towards each 

 other. In this case the magnets (M^, M^) and lines of force repel each other, as indicated by the darts (ij, tj). n^, n.^, The north poles 

 of the magnets ; s^, .Sj, the south poles of the magnets ; j, indifferent or zero point caused by mutual and equal repulsion. At this point 

 a short magnetic needle is not influenced or exposed to any directive agency. This figure shows that the lines of force proceeding from 

 each north pole (jij, n.^) are bent back on encountering each other, and curve sharply round, and at points become more or less parallel. 



Pig. 2.— This figure is the opposite of Fig. 1. In this case the north and south poles of two magnets (ilfj, M^) are directed towards 

 each other, with the result that the linos of force are attracted or drawn towards each other as indicated by the darts (I'l, ij)- ^v "'Z' 

 the north poles of the magnets ; Sj, s.^, the south poles of the magnets. This figure shows that the lines of force proceeding from the 

 pole »i, of the one magnet (Mj), Ijend round and unite with the lines which terminate in the pole s^ of the other magnet (M.^). In the 

 interspace between the unlike poles of the two magnets (re,, s^), tbe lines of force are crowded together and very little beiit or curved 

 (j). In the more outlying parts of the field they are more spread out, and pass in wide curves from one pole to the other. The lines 

 of force are less spread out and curved than in Fig. 6, Plate Iv. ; a circumstance due to tension along the lines of force and to cross 

 pressure. 



Figs. 3 and 4 are produced by two bar-magnets of equal power fixed in a vertical position at different distances from one another ; 

 the poles being like or unlike according to arrangement. In Fig. 3 the poles are unlike (n, s), whereas in Fig. 4 they are like (n, n). 

 In Fig. 3 the iron-filings radiate from the unlike poles («, s), and bend towards each other in graceful curves between the poles ; the 

 curves becoming larger as the mesial line between the poles is receded from. In Fig. 4 the iron-filings radiate, but the curves formed 

 by tliem in the interspace (j) between the poles (re, n) are absent ; a circumstance due to like poles and the lines of force proceeding 

 therefrom repelling each other. The rule in such cases is, "Like poles repel and unlike poles attract." The interspace (j) forms an 

 indifferent zone. In Figs. 3 and 4 the action and reaction of the poles on each other is very marked. The figures illustrate the effects 

 produced by attraction and repulsion respectively, and how plant and animal structures and movements may be modified. 



Fig. 5. — Lines of force figure produced by the action of the unlike poles of two bar-magnets adjacent to one another on iron-filings. 

 The lines of force proceeding from the poles (w,, s,) of tlie one magnet bend round until they reach the poles (fjj, %) of the other magnet. 

 They unite with the lines proceeding from the latter poles, and establish a connection between the t-wo bars — the lines of force near 

 the poles being almost straight. In the space outside the magnet the lines of force are bent, and form larger and larger curves, because 

 of pressure perpendicular to their direction, i',, i.^, The different zones of the magnetic field ; j, region where the lines of force 

 are absent. 



In the case of two bar magnets with like poles adjacent to one another the lines of force which proceed from the poles (n,, Sj) of the 

 one magnet curve back on meeting the poles (wj, h) o^ ths other magnet, and in so doing largely avoid the space between the two 

 magnets and run parallel. 



The lines of force in Fig. 5 very closely resemble similar lines seen in centres of development in crystals, and lines of growth in 

 plants and animals. 



Fig. 6. — Lines of Ibrce figure obtained in a plane perpendicular to a rectilinear conductor. In the middle of the picture is seen the 

 circular section (c) of the cylindrical wire ; the iron-filings having arranged themselves in complete rings embracing it concentrically. 

 There are no points at which the lines of force originate or terminate ; in other words, there are no sources or sinks in the field. The 

 force exerted is greatest in the vicinity of the wire, where the iron-filings are aggregated and form a dark ring : the force decreases as 

 the wire is receded from, the rings of iron-filings becoming larger and more faint in a direct ratio to the distance. " All the lines of 

 force due to a current in a straight wire lie upon cylindrical surfaces whose axis coincides with that of the wire." This figure has an 

 obvious bearing on the formation and growth of all concentric structures, as witness the cross sections of many crystals, the stems, 

 branches, and other parts of plants, and the bones, teeth, and other parts of animals. 



Fig. 7.— Shows a lines of force diagram with a current flowing round a ring. The arrangement consists of a horizontal sheet of 

 paper with iron-filings, through which a thick copper circular wire, the ends of which are not quite in contact, runs in a perpendicular 

 direction. When a current is sent round the wire in the direction (-1- to -) indicated by the darts, the concentric lines of force seen 

 within and without the circular copper wire are obtained ; they are more crowded within than on the outside of the wire. The actual 

 lines made by the iron-filings are given at Fig. 8. 



Fig. 8. — Lines of force figure produced by a current sent along an open copper ring running perpendicularly through a horizontal 

 sheet of paper with iron-filings. According to Professor Bbert, " The direction in which the lines of force are to be positively reckoned 

 may be easily deduced from the positive direction along the conductor (axis of force). If this latter direction is considered clock -wise 

 from our jioint of view, the lines of force will encircle the left limb of the wire clock-wise as seen from above. These lines then as 

 they spread through the ring are directed towards the observer, the same being true of the lines which encircle the ri^ht limb of the 

 wire, and, in fact, all the lines of the system. Thus, a translation in the direction of the magnetic force, combined with a rotation in 

 the sense of the current, would constitute a right-banded screw motion. "We may say tliat a current and the lines of induction which 

 it produces embrace one another right-handedly" (Ebert, Part I. p. 193). 



This lines of force figure bears a general resemblance to cross sections of various parts of plants and animals. 



Fig. 9. — Lines of force figure produced by the action of a conducting wire and the south pole of a bar-magnet on iron-filings The 

 presence of the south pole (.s) of the magnet deforms or disarranges the field of concentric magnetic lines of force produced by the 

 action of the conductor (c), and gives rise to a spiral arrangement of the iron-filings. Contrast with Fig. 6, where the magnetic field is 

 undisturbed. The current passes through the plane of the picture from below upwards. " Of the lines of force which densely encircle 

 the cross section of the conductor (c) a considerable number end at the 'sink' s (south pole of bar magnet). Here they leave the open 



