548 



NA TURE 



[August 9, 1894 



becomes, therefore, a matter of interest to disentangle the two 

 constituents of local disturbances : and there u one question to 

 which I think an answer might be obtained without a greater 

 expenditure than the importance of the investigation warrants. 

 Are the local variations in secular change waves which move 

 from place to place, or are they stationary fluctuations, each of 

 which is confined to a limited area beyond which it never 

 travels? Thus, if the annual decrease in the declination is at 

 one time more rapid at Greenwich than at Ivew, and live years 

 afterwards more rapid at Kew ihan at Greenwich, ha-> the 

 maximum of rapidity passed in the interval through all interven- 

 ing places, or has there been a dividing line of no change which 

 has separated two districts which have perhaps been the scenes of 

 independent variations? The answer to this question is, I lake 

 it, outside the range of our knowledge now, bui if the declination 

 could bj determine! several tim;s annually at each of a limited 

 number of stations in the neighbourhood of London, to this 

 inquiry, at all events, a definite answer would soon be 

 furnished. 



There are two other lines of investigation which I hope will 

 be taken up sooner or later, for one of which it is doubtful 

 whether the United Kingdom is the best site, while the other is 

 of uncertain issue. 



If, however, it be granted that the principal cause of local 

 and regional magnetic disturbances is the magnetisation by the 

 earth's field of magnetic matter concealed below its surface, the 

 question as to the nature of this material still remains to be 

 solved. Is it virgin iron or pure magnetite, or is it merely a 

 magnetic rock of the same nature and properties as the basalts 

 which are found in Skye and Mull ? There is, of course, no <) 

 priori reason why all these different materials should not be 

 active, some in one place and some in another. 



As regards the United Kingdom I have, both in a paper on 

 the Permeability of Magnetic Rocks and in the description of 

 the recent survey, made calculations which tend to prove that, 

 if we suppose that the temperature of the interior of the earth 

 is, at a depth of twelve miles, such as to deprive matter of its 

 magnetic properties, and if we further make the unfavourable 

 assumption that down to that limit the susceptibility is constant, 

 the forces which are observed on the surface are of the same 

 order of magnituie as those which could be produced by large 

 masses of oidinary basalt or gabbro. It would not, huwever, 

 be wise to generalise this result, and to assume that in all places 

 regional disturbances are due to basic rocks alone. 



We know that local effecis are produced by iron ore, for the 

 .Swedish miners seek for iron with the aid o( the magnet, and 

 in some other cases magnetic disturbances of considerable r.inge 

 arc so intense as to suggest that material of very high magnetic 

 permeability must be present. 



I( the concealed magnetic matter were iron, and if it were 

 present in large quantity, it is evident that the results of ex- 

 periments with the magnetometer and dip circle might 

 be supplemented by observations made with the plumb-line or 

 pendulum. In such a case the region of magnetic disturbance 

 would aUo be a region of abnormal gr.ivitaliunal attraction. 

 An account of a suggested connection between anomalies of 

 these two kinds occurring in the same district has lately been 

 published by Dr. I'ritsche.' 



Observations made about thirty years ago by a former director 

 of the Agronomical Observatory in Moscow led to the con- 

 clusion that throughout two large districts to the north and 

 south of that city the plumb-line is deviated in opposite direc- 

 tions. The deflections from the vertical are very considerable, 

 aod indicate a relative defect in the attraction exerted by the 

 rocks in the neighbourhood of Moscow itself, and the sugges- 

 tion ha* been made that there is cither a huge cavity — a bulible 

 in the caithcrust — under the town, or that the matter bci>eaih 

 it is les* dense than that which underlies the surface strata on 

 eiili' r ..I.- at a distance of ten or twelve miles. 



a« 1853, Captain Mcyen made magnetic observa- 

 ti'> >o determme whether the same district is also the 



seat i.f any magnetic irregularity. ilis stations were hariily 

 •ufhciently numcrnus to lead to decisive results, but the magnetic 

 elc Illy been mcasurcil by Or Kritschc at thirty- 



OT' 'y miles of Moscow The experiments were 



all '■ ^..jven ilayit, so that no correction for secular 



change is required. Tney indicate a locui of magnetic 



' ' ' ' -fi li€i Moikati uod ihrc Be/le- 

 dliliH dt la Sacim Imftr. dct 



A'.. 



NO 1293. VOL. 50J 



attraction running through Moscow itself. South of the town 

 the disturbance again changes in direction so as to show either 

 that repulsive forces are in play, or that there is another 

 magnetic ridge line still further to the south. Dr. Fritsche 

 thinks that these observations explain the gravitational' 

 anomalies without recourse to the someixhat forced hypothesis 

 of a vast subterranean c.ive. He assumes that there is a con- 

 cealed mass of iron, which approaches near lo the surface at 

 Moscow, and al-o along two loci lo the souih and north of the 

 city. He attributes the magneiic irregularities lo ihc attraction 

 of ihe central iron hill, the defleclions of the plumh-line to the 

 flanking masses. It is perhaps not inconceivable that such 

 results might follow in a special case, but wiihout the support of 

 calculation it certainly appears that the mngnetic experiments 

 point to ihe existence of the principal attracting mass under the 

 town. This is in fact the arrangement shown in the figure with 

 which Dr. Kritsche illustrates his hypothesis. If this is so, the 

 theory would prima fiicie seem to require that the bob of a 

 plumb-line should be attracted towards and not — as is actually 

 Ihe case — away from the centre of the magnetic disturbance. 

 On the whole, then, though the coexistence of l.irge magnetic 

 and gravitational disturbances in the same place is suggestive, I 

 do not think that they have as yet been proved to be different 

 effects of the same hidden mass of magnetic matter. 



In a few weeks an International Geodetic Conference will 

 meet at Innsbruck, at which the Royal Society will be repre- 

 sented. It is, I believe, intended to extend the detailed in- 

 vestigation of the relations between Ihe nature of the earth's 

 crust and the gravitational and magneiic forces to which it gives 

 rise. \Vc may therefore hope that soeci.al attention will before 

 long be given to localities where both may combine to give 

 information as to facts outside the range of the ordinary methods 

 of geology. 



The second phenomenon on which more light is desirable, is 

 the permanent magnetisation of magneiic rocks. It is known 

 that fragments of these are strongly but irregularly m.agnetised, 

 but that the effect of very large masses at a distance appears to 

 be due lo induced rather than to permanent magnetism. There 

 are three questions to which I should like an answer. Are 

 underground masses of magnelile ever permanently magnetised ? 

 Are large areas of surface ma-ses, say a few hundred square 

 yards in extent, ever permanently and approximately uniformly 

 magnetised in the same sense ? Is there any relation between 

 the geological age and the direction of the permanent magnetism 

 ol magnetic rocks? 



Inquiries such as these can only be taken up by individual 

 workers, but I venture to think that the comparison of the 

 observatory instruments and the fluciuatlons of secular ch.inge 

 outside the observatories could best be investigated under the 

 auspices of a great scientific society. The co-operation of the 

 authorities of the observatories will no doubt be secured, but 

 it is most important that the comparison should in all cases be 

 made with one set of instruments, and by the same methods. 

 Whether the British Association, which for so long inana;^ed a 

 magneiic observatory, may think that it could usefully inaugu- 

 rate the work, it wouM be improper for me in a presidential 

 address to forecast. Who does it is of less importance than 

 that it should be done, and I cannot but hope that the argu- 

 ments and instances which I have to-d.iy adduced may help to 

 bring about not only the doing of the work, but the doing of 

 it quickly. 



SECTIO.V B. 

 CHEMISTRY. 



Opesi.ng .Vddkiss 1)y Prof. H. B. Dixon, M..\., !•'. R.S., 

 Preside.nt of the Sectio.v. 



"An Oxjoni 'ichool of Chemists." 



It has been said, and no doubt with truth, thai few Presidents 

 of Sections start writing an address without referring to that of 

 their predecessor who held office on the last occasion when the 

 Association met in the same cily. By such reference each new 

 President gains the advantage of many points of perspective 

 and contrast ; for in the interval a generation of workers has 

 passed away, and the Kasl new thing of the old meeting is the 

 ancient instance of to-day. In ihe present case I turned to 

 the Report of i860 with a lively hope of drawing inspiration 

 from it i for my predecessor at the last Oxford meeting was no 



