222 



NATURE 



XJan. 21, 1875 



that which attributes a permanent loss of magnetism only 

 to an increase of temperature. 



We now come to the most valuable part of the work, 

 or those chapters which treat of terrestrial magnetism. 

 Dr. Lloyd tells us in his preface that the course he has 

 pursued has been '' to present the results obtained at a 

 single station, i.e. Dublin, at which all the general fea- 

 tures of the phenomena belonging to the middle northern 

 latitudes were fully developed, and to supplement the 

 information by the results of observation at places widely 

 removed from the former in geographical position, as well 

 as in their relation to the sun's daily and yearly courses.'' 



Dr. Lloyd adds that " he has not entered upon the 

 interesting speculation connected with the physical causes 

 of the phenomena, further than to reprint a paper, pub- 

 lished by himself many years ago, in which the agency of 

 the sun and moon are shown not to be due to their direct 

 operation as magnetic bodies." And he concludes his 

 preface by remarking that " the electrical earth-currents 

 must have their elTect upon the magnetical variations 

 recorded in our observatories, whether they be the sole, or 

 only a co-operating cause." 



Are we justified in our inference, from the concluding 

 observation, that Dr. Lloyd has to some extent modified 

 his views with regard to the importance of these earth- 

 currents .'' for, if we are not mistaken, he was at one time 

 inclined to attribute the daily variations of terrestrial 

 magnetism to their operation. 



Speculations regarding the causes of the phenomena 

 of terrestrial magnetism may be divided into two classes : 

 firstly, those which attempt to account for the magnetisa- 

 tion of the earth ; and secondly, those which only pretend 

 to account for the changes taking place in this magneti- 

 sation. It is from the latter point of view that we would 

 now venture to make a few remarks. 



Let us assume, to begin with, that nothing is definitely 

 known regarding the causes of these changes. Let us 

 next endeavour to enumerate and discuss the various 

 agencies we know of which may be conceived to take a 

 part in producing these phenomena, in the hope that by a 

 pieliminary trial of its kind we may, perhaps, light upon 

 the true cause, even although the evidence at our disposal 

 be insufficient to give certainty to our suspicions. 



In the first place, we may take it for granted that 

 neither the sun nor the moon can cause the changes in 

 terrestrial magnetism, which they are known to produce, 

 by virtue of their direct magnetic influence. 



This point has been sufficiently discussed both by Dr. 

 Lloyd and by Mr. Chas. Chambers, and the conclusion 

 to which both of these magneticians have arrived is, that 

 the magnetic effects caused by the sun and moon are not 

 due to their direct operation as magnetic bodies. 



Let us take the sun and confine ourselves in the mean- 

 time to the daily variations which he causes. Now, first 

 of all, it is clear that these are not due to any kind of 

 tidal action of the sun, or to the indirect consequences of 

 such an action, inasmuch as there is only one maximum 

 and one minimum in the day. 



The only other known way in which the sun can affect 

 the earth is through his heat ; and starting with the 

 assumption that the earth /j- a magnet, no matter how or 

 •why, let us next enumerate the various ways in which 

 the heat of the sun may possibly affect the earth. 



In "dx^ first f)lace, it might influence the magnetic pro- 

 perties of that medium (the air) which surrounds the 

 earth and any suspended magnet. 



Or, secondly, it might produce a temperature effect 

 upon the earth itself considered as a magnet. 



Or, ihirdly, it might be conceived to generate thermo- 

 electric currents in the earth. 



Or, fourthly, it might cause the motion of conducting 

 bodies across the earth's lines of magnetic force. 



The first of these is the hypothesis of Faraday ; and 

 while the chaage produced by heat in the magnetic quali- 

 ties of the atmosphere cannot be without its influence, 

 yet it is, we believe, the universal opinion of magneticians 

 that this change cannot account, either in magnitude or 

 law, for the somewhat considerable daily variation. The 

 diurnal change produced by the sun's heat in the mag- 

 netic condition of the crust of the earth must be still 

 more insignificant, and may be at once dismissed. 



Our attention is thus concentrated" on the third and 

 fourth of the above possible causes, one of which we may 

 perhaps expect to account for the daily variation, unless 

 this be due to some cause of the nature of which we are 

 entirely ignorant. 



It is now well known that what are called earth-cur- 

 rents are of very frequent, if not continuous occurrence, 

 and we are indebted to the present Astronomer Royal for 

 an experiment made with the view of ascertaining the 

 nature of the relation between these fcurrents and the 

 changes of terrestrial magnetism. He set up certain 

 wires on the Croydon and Dartford lines, which gave him, 

 by means of a self-recording arrangement, a continuous 

 record of the strength and duration of these earth-cur- 

 rents, and the following is the conclusion which he has 

 derived from the discussion of these observations :— 



" Neither in magnitude nor in law are these inequa- 

 lities consequent on the galvanic currents competent to 

 explain the ordinary diurnal inequalities of magnetism." 



In fact, there is some'reason to regard these currents 

 rather as the effects than as the causes of magnetic 

 changes, that is to say, to view them as secondary 

 currents ; and the author of this notice has shown in a 

 paper, published in the Transactions of the Royal Society 

 of Edinburgh, that these earth-currents are strongest at 

 those periods of the day when the change in terrestrial 

 magnetism is most rapid — a result which would follow if 

 the earth-currents were secondary currents due to mag- 

 netic changes. Our attention is thus drawn to the fourth 

 hypothesis as the only remaining conceivable cause of 

 magnetic changes, unless these are caused by something 

 of which we are entirely ignorant. 



It is known that Faraday tried to detect induction 

 currents in the Thames, supposing that these might be 

 caused by the carriage of a conducting liquid across the 

 earth's lines of magnetic force, but found no positive 

 result. Sir W. Thomson afterwards made a proposal to 

 test the idea by tides in the English Channel, but we do 

 not think this has ever been carried out. He also dis- 

 cussed to some extent the part which may be played in 

 the phenomena of terrestrial magnetism by moving con- 

 ductors. 



But to return to the fourth hypothesis. In the first 

 place, let us ask ourselves the question, Under what cir- 

 cumstances can the convection currents generated by the 



