December 23, 1897] 



NA TURE 



183 



vhich is many times larger than that of the secular orbits already 

 depicted (Fig. 5). The upward and downward movement of 

 the needle is much smaller than the displacement east and west, 

 so that an elongated figure is produced, but the interesting 

 point is that it is not a closed curve ; the two ends do not meet, 

 but are separated by a very appreciable interval. It would, of 

 course, be an easy explanation of this fact, if we could attribute 

 it to the secular change. Just as the moon, though at the end 

 of a month in the same position with respect to the earth as it 

 was at the beginning, is much further advanced in the earth's 

 orbit, so the diurnal magnetic variation must accommodate 

 itself to the larger secular movement of which it is a subordinate 

 part. But this explanation alone will not suffice. It is true that 

 during the quiet days the movement of the needle in its secular 

 path continues, but there is a good deal of evidence to show that 

 it is of more than average speed. This is especially true of the 

 horizontal force, which is gradually increasing and increases 

 with remarkable rapidity on quiet days. Hence the secular 

 movement appears to be checked by the storms. The com- 

 paratively rapid progress which has been made in quieter times, 

 being retarded and even reversed during the periods of irregular 

 motion which I have described. 



It is true that General Sabine many years ago showed that 

 magnetic storms do not act equally in both directions upon the 

 needle, and that thus the phenomena which I am now describ- 

 ing can hardly be said to be recently discovered ; but the method 

 of presenting it which has been adopted by Dr. Chree, and 

 which I have slightly modified by including in the diagram the 

 variations both of declination and of dip, certainly places the 

 facts before us in a novel and a striking light. Of what the 

 cause of the sudden check which the needle receives during the 

 magnetic storms may be, we can as yet say nothing. It is one 

 of the puzzles which has yet to be unravelled. 



The last point to which I will refer is one upon which more 

 definite results have been obtained. Terrestrial magnetism is 



Fig. 5. 



connected with phenomena which occur in the sun, with the 

 Aurora Borealis in the upper atmosphere, and with the earth 

 currents which traverse the soil. I have now to draw your 

 attention to its relation to geology. 



It has long been known that just as the great secular variation 

 of the magnet is accompanied by minor diurnal changes, so the 

 large alterations in the direction of the compass and dipping 

 needle, which are observed when we move from place to place 

 on the surface of the earth, are affected by irregularities which 

 are apparently due to purely local causes. Thus the declination 

 is greater in Ireland than in England ; but the increase is not 

 uniform as we pass from one country to the other. In fact in 

 some districts an abnormally large increase is followed by a 

 decrease. 



These curious inequalities must be due to local disturbing 

 forces, and the large number of observations which have been 

 made in this country have enabled us to determine with more 

 than usual accuracy the magnitude and direction which the mag- 

 netic forces would assume if they were undisturbed by any local 

 cause, and from the difference between things as they then 

 would be and things as they actually are, we can calculate the 

 magnitude and direction of the disturbing forces themselves. 

 When these are represented on a map, it is found that there are 

 large districts of the country in which the disturbing horizontal 

 forces act in the same direction ; in one region the north pole of 

 the needle will be deflected to the east, in another to the west, 

 and, as we pass from one of these districts to the other, we 

 always find that at the boundary the downward vertical force on 

 the north pole of the needle reaches a maximum value. We are 

 thus able to draw upon the map lines towards which the north 

 pole of the needle is attracted. It is found that the exact 

 position of these can be determined with considerable accuracy, 

 and that the lines can be traced without any possible doubt 

 through distances amounting, in some instances, to a couple of 

 hundred miles. The key to this curious fact is probably fur- 



nished by observations in the neighbourhood of great masses of 

 basalt or other magnetic rocks. If these were magnetised by 

 the induction of the earth's magnetic field, the upper portions of 

 them would in this hemisphere attract the north pole of the 

 needle ; and it is found that where large masses of basalt exist, 

 as in Antrim, in the Scotch coal-fields, in North Wales, and 

 elsewhere, the north pole of the needle is, as a matter of fact, 

 attracted towards them from distances which may amount to 

 fifty miles. The thickness of the sheets of basalt is in most 

 cases too small to furnish a complete explanation of the ob- 

 served facts, but it is quite possible that these surface layers 

 of magnetic matter are merely indications of underground pro- 

 tuberances of similar rocks from which the surface sheets have 

 been extruded. At all events, there is no possible doubt of the 

 fact that where large masses of basalt occur, the north pole of 

 the needle tends to move towards them. 



There are other regions where the attractions are manifest, 

 but where, nevertheless, no magnetic rocks occur upon the sur- 

 face ; but it is most probable that the cause is the same, and 

 that it is due to the mere accident of denudation that in one 

 case we can, and in the other we cannot, point to the magnetic 

 rocks to which the anomalous behaviour of the compass is due. 

 If this be so, it is certainly interesting that magnetic observations 

 should enable us to penetrate to depths which the geologist can- 

 not otherwise reach, and that the lines which we draw upon the 

 surface of the map, as those to which the north pole is attracted, 

 may, in fact, roughly represent the ridge-lines of concealed 

 masses of magnetic rocks, which are the foundations upon 

 which the deposits studied by the geologist have been laid. 



There is some ground for thinking that if these great under- 

 ground wrinkles exist, they have affected the rocks which are 

 superposed upon them, especially those which are of a com- 

 paratively early date. As a general rule, if older rocks appear 

 in the midst of newer ones, the pole of the magnet will be 

 attracted towards the protruding mass ; but this rule holds good 

 only of the rocks of carboniferous or pre-carboniferous age, and 

 does not apply to later deposits. As a striking example, I may 

 remind you that the Pennine Range — which is sometimes called 

 the " Backbone of England" — is a mass of millstone grit rising 

 amid younger rocks. Down this a well-marked magnetic ridge- 

 line runs. Similarly, in the neighbourhood of Birmingham, the 

 Dudley and Nuneaton coal-fields are surrounded by more modern 

 deposits. A curious horse-shoe shaped ridge line connects these 

 two, and then runs south to Reading, which is, magnetically 

 speaking, one of the most important towns in the kingdom. 

 East and west from Dover to Milford Haven, and then across 

 the Irish Channel to Wexford, runs a ridge of the older rocks 

 called by geologists the Palaeozoic Ridge, concealed in many 

 places by newer deposits. Hollowed out in this are the South 

 Wales and Forest of Dean coal-fields, and in another hollow 

 within it lies the coal which has recently been discovered at 

 Dover. Closely following this protruding mass of the older 

 rocks is a magnetic ridge-line which passes through Reading, 

 and we thus have a magnetic connection between the anticlinals 

 of Warwickshire and the Palaeozoic Ridge. From the neigh- 

 bourhood of Reading also another magnetic ridge-line runs 

 southwards, entering the channel near Chichester. M. Moureaux, 

 who with most untiring energy has for many years been investi- 

 gating single-handed the magnetic constitution of France, has 

 discovered the continuation of this line on the French coast 

 near Dieppe, and has traced it through the north of France to 

 some fifty miles south of Paris. The energy which is now being 

 displayed by magnetic surveyors in many countries will, no 

 doubt before long, prove that the network of these magnetic 

 ridge-lines is universal ; and the relations between them and the 

 geological conformation of the countries in which they lie will 

 be so studied that our inductions will be based upon an adequate 

 knowledge of facts. 



This, at all events, we may hope, that amid the flux and 

 change of magnetic forces with which we have so largely been 

 occupied, we may have found in these ridge-lines physical 

 features of the country as permanent as the hills themselves. 



And now that I draw near to the end of my lecture. I cannot 

 but feel that it has to a certain extent been wanting in organic 

 unity. It is sometimes possible to unfold the story of a scientific 

 advance in strictly dramatic form. The question posed, the 

 oracle consulted, and the answer given, might well form the 

 titles of the three acts in the modern miracle-play of scientific 

 discovery. But the drama has its conventions, and even those 

 authors who boast of their realism, too often falsify by over- 



NO. 1469, VOL. 57] 



