August 1, 1900.] 



KNOWLEDGE. 



187 



I want now to apply the foregoing representations 

 to the explanation of the aetion of a telegraph wire a.s 

 employed in ordinary telegraphy. What happens here 

 is that a magnetic field at the sending station is made 

 to excite a magnetic field at the receiving station with 

 comparatively small loss. The wire makes it pos.sible 

 to produce this secondary field in any place desired. 

 To understand how this is to be explained we will return 

 to the consideration of the rack and train of wheels, 

 but in the first place assume for greater simplicity that 

 the wire is a perfect conductor. The rack must there- 



0^ 



1#,^ 



0m 



mm 



#0 



Via. 5. — Lodge's Model illustrating a Section of a Magnetie Field. 



Fig. 6. — Lodge's M>riel illusti-ating a Section, taken throiin'i t'n- 

 wire, of a Wire earr^'ing an Electric Current with its Magnetic 

 Field. 



Fia. 7. — Illustrating a Magnetic Vortex Whirl encircling a Wire 

 earrring an Electric current. 



From Lodge's " Modern Views of Electricity." 



fore be removed and replaced by a smooth rod, so that 

 the magnetic spin may cease at its surface and transmit 

 no energy into the wire. Assume at the same time that 

 the rotation of the wheels is in some manner maintained 

 just as if the rack were being pushed along. Then in 

 the bounding surface of the rod representing the con- 

 ducting wire there exists the state of slip, which has 

 been shown to correspond with an electric current, and 

 it will be seen that the function of the rod or conductor 

 is simply to provide a space free from the magnetic 

 wheelwork, so as to allow of the free rotation in opposite 

 directions of the wheels on the opposite side of any 

 longitudinal section through the rod. If the space were 

 not thus kept free the wheels would interlock, and the 

 only magnetic field would be the ordinary state of spin 

 about the lines of force, rapidly diminishing in intensity 

 as the distance from the battery or other source jf 

 energy is increased. With this space, however, kept 

 free by means of the perfectly conducting wire or smooch 

 rod in the model, there will be an intense magnetic field 

 everywhere immediately in the neighbourhood of the 

 wire and diminishing in intensity as the distance from 

 the wire increases. 



All along the wire there will be, in fact, vortex whirls, 

 as shown in Fig. 7, where B is a conductor carrying 

 a current the direction of which is indicated by the 

 arrow. The direction of spin of the positive whirls is 

 shown by the curved arrows. All that is required in 

 order to enable the wire to act in this manner is to 

 have some arrangement capable of exciting vortex whirls 

 about some portion of the wire, which must form a 

 closed circuit, and these vortex whirls will then travel 

 along the wire and produce their effect at the distant 

 stations. These whirls are not found in the wire itself, 

 but in the insulating sheath, so it will be seen that the 

 wire transmits nothing, but only directs the energy on 



its way by holding apart the mutually opposing wheel- 

 work of the insulator. 



In practice the wire is not, of course, a perfect con- 

 ductor, but the effect of this is merely that the slip on 

 its surface is imperfect. Some of its own wheelwork 

 is therefore set in motion, except along the axis of the 

 wire. Two distinct results follow from this. In the 

 first place, the frictional slip in the imperfec-t conductor 

 causes a dissipation, into heat, of some of the energy 

 supplied, and therefore only a portion of the initial 

 energy at the sending station is transmitted to the 

 receiving end. In the second place, every time the wheel- 

 work is started, there will be a certain delay, increasing 

 with the diameter of the wire, and which will also be 

 comparatively large if the wheelwork of the conductor 

 is very massive, as would be the case if an iron wire 



were employed. 



* 



THE LAND OF THE BASTIDES. 



By Grenville A. J. Cole, m.r.i.a., f.g.s., Professor of 

 GroJoc/;/ in the Royal College of Science for Ireland. 



At a time when the authority of a central power was 

 beginning to be felt, and when the free cities were being 

 replaced by villes royales, dependent on the King of 

 France, a number of fortified posts sprang up in the 

 wilder country of the south, occupied by " king's men," 

 and essentially military in design. From 1250 to 

 1350 A.D., these boroughs continued to accumulate, and 

 to this day there are twenty-seven towns in central and 

 southern France that are called simply La Bastide. 



The original " bastida " of Provence may have had 

 a high antiquity. It was a fortified farm, like some of 

 those that are still inhabited on the flanks of the Juras, 

 or in the English Pale round Dublin. The positions 

 occupied by several of these strongholds can hardly have 

 been neglected in Gallo-Roman days ; but the bastide, 

 a.s it now appears, dates mainly from the fourteenth 

 century. Far- away, you may see the yellow wall, with 

 red-tiled roofs above it, and here and there a round 

 tower at a comer, crowning a spur of the valley-side. 

 Five miles on, you may spy another, and one more, 

 perhaps, across the river, set against the pale hot purplo 

 of the sky. The roads occasionally desert the alluvium, 

 and climb from one bastide to the next, passing in at a 

 gate decked with some Palladian ornament, round the 

 town between the ramparts and the houses, and out 

 again into the blaze of sunlight, the permeating sun- 

 light of Provence. 



The landscape is a mixture of yellow and grey- 

 green; the hillsides crumble in summer into brown 

 and yellow earth ; the quarries of soft stone arc 

 yellow ; a yellow distemper has even seized upon 

 the houses. The vines, on their posts and Roman 

 trellises, are heavy with grapes, and dull with 

 wind-bome dust ; the trees along the road, planted 

 by a benevolent government, have struggled through 

 a joyless youth into a middle age of inutility. 

 The hot air blows from Narbonne and the Mediter- 

 ranean, and the pink haze hides both the Montague 

 Noire and the Pyrenees. 



In this broad valley, where the head waters of tho 

 Gascon rivers almost touch those of the shorter eastf.rn 

 system, we see Fanjeaux, a fortress on the scarp, xnd 

 Montreal, with its tall Italian campanile, and finally, 

 grey and unbelievable, spreading in the distance like 

 a dark wood along its plateau, the Cite of Carcassonne, 

 roofed and towered as Froissart left it, par excellence 

 the great bastide. 



