December i6, 1897] 



NA TURE 



161 



accurate and exleiisive, we can determine the relative propor- 

 tions of those parts of the force which are due to causes wholly 

 external or internal respectively. It is only lately that a further 

 attempt has been made to discover whether, in addition to these, 

 currents from earth to air and from air to earth also exist. The 

 credit of this attempt is due to Dr. A. Schmidt, who, taking the 

 most recent and the most accurate facts at his disposal, deduced 

 from them the conclusion that about one-fortieth part of the 

 magnetic force is due to causes wholly external to the earth, and 

 that a slightly larger fraction is produced by vertical currents ; 

 the origin of the remaining thirty-eight fortieths being traced to 

 internal causes only. 



And now it becomes necessary to say a few words as to the 

 method by which the vertical earth-air currents maybe detected. 

 If we could perform the impossible operation of severing the 

 north pole of a magnet from the remainder without immediately 

 producing poles of the opposite kind in the broken fragments, 

 the isolated pole thus manufactured would be urged northwards 

 by the magnetic forces which are in play near the surface of the 

 earth. If therefore a traveller were to carry such a pole with 

 him, he would be assisted when going northwards, retarded 

 when returning to the south. If the tour ended at the starting- 

 point, the advantage gained when moving in one direction 

 would in general be exactly compensated by the disadvantage of 

 being compelled to oppose the magnetic forces during the 

 remainder of the journey. 



To this rule there is one exception. If the migrations of the 

 magnetic pole carried it round an electrical current, so that its 

 course passed through the circuit in which the current flows, as 

 a thread might pass through a ring, and if the route finally led 

 back to the starting-point without again passing through the cir- 

 cuit of the current, the exact equilibrium of loss and gain would 

 be destroyed, and when the journey was over the wandering 

 pole would either have added to or drawn upon any store of 

 energy which it might at first have possessed. 



Whether the result would be a loss or a gain would depend 

 upon the direction in which the journey was performed relatively 

 to the direction of the current. On this point it is unnecessary 

 to dwell. Suffice it to say that if the amount of the loss or gain 

 experienced by a given pole is known, the magnitude and 

 direction of the current, whose circuit had been traversed, can 

 be calculated. The result would not be affected by whether the 

 current flowed from all parts of the district which the path of 

 the pole had encircled, or was confined to a few points only ; 

 the total flow would be registered without reference to how it 

 was distributed. If some of the currents flowed in opposite 

 directions the excess of one set over the other would be 

 measured. 



If now a current passes at a certain point from earth to air 

 it must return from air to earth elsewhere, completing the circuit 

 through the soil. The course of the unburied portion may be 

 regarded as an aerial arch, and from what has been said it will 

 be evident that if a magnetic pole were carried round a leg of 

 this arch the circuit of the current would be pierced, and the 

 total upward or downward flow would be determined. The 

 experiment, as thus described, is impossible, but, by an appro- 

 priate method, we can determine the force which would be 

 exerted at any point on the detached north pole of a magnet of 

 given strength, and, if this be known for a sufficient number of 

 points on the path, we can calculate what the result would be 

 if the imaginary conditions of the journey could be realised. 



The calculations of Dr. Schmidt as to the existence of earth- 

 air currents were based upon this principle, and were applied to 

 the earth as a whole. Their general accuracy has been con- 

 firmed by Dr. Bauer, who supposed the hypothetical isolated 

 magnetic pole to be carried along lines of latitude right round the 

 earth. It, for instance, the journey were made along latitude 

 51^°, beginning and ending at London, the resulting work would 

 show the total amount of the currents which traverse the 

 northern portions of the Northern Hemisphere between that 

 latitude and the geographical pole. If the same operation were 

 repeated, say on latitude 45°, a similar result would be obtained, 

 and the difference between the two would give the average flow 

 of the currents which traverse the, surface of the earth between 

 these two latitudes. 



Of course, it must be remembered that by such a calculation 

 we can only arrive at a mean result. If, for instance, we had 

 proved that between these latitudes there was, on the whole, an 

 upward current, it would by no means follow that at all points 

 on the vast surface included between the selected boundaries 



NO. 1468, VOL. 57] 



the currents were flowing from below to above. The meaning 

 of the result would be that, within the region considered, the 

 upward were stronger than the downward currents, and that, if 

 the excess were uniformly distributed over the whole of the 

 surface to which the calculation applied, an average current of 

 such and such a magnitude would be produced. 



Turning from the method of detecting the vertical currents to 

 the question as to whether they exist, there are, apart from the 

 calculations of Schmidt and Bauer, some experimental and 

 tlieoretical reasons which support an affirmative answer. We 

 know that earth currents traverse the soil beneath us. The 

 Aurora is evidence of electrical discharge in the atmosphere. It 

 is conceivable that there are cross connections between these 

 two systems. Again, if the immediate surroundings of the 

 earth are electrically conducting, the mere rotation of the huge 

 magnetic mass of the earth itself would cause the production of 

 currents which at some points would flow out of, and at others 

 would flow into the surface. The late Prof Hertz calculated 

 the forms of the paths of such currents for the case of a uni- 

 formly magnetised sphere rotating about its magnetic axis, and, 

 though the fact that the magnetism of the earth is irregularly 

 distributed forbids us to apply his calculation directly to the 

 globe, yet the principle holds good, though the distribution of 

 the currents would be more complex. Dr. Bauer has deduced 

 from the calculations already 

 referred to the average direction 

 of flow between different lati- 

 tudes. 



The result is shown in Fig. i. 

 The directions and magnitudes 

 of the supposed vertical currents 

 are indicated by arrows, and 

 points at which there are no 

 such currents occur at lat. 43° N. 

 and 40' S. 



Up to this point, therefore, 

 the argument seems all in favour 

 of the actual existence of cur- 

 rents from earth to air, but the 

 results of calculations such as 

 these must be accepted with very 

 great caution. Our knowledge 

 of the magnetic state of the 

 earth is very imperfect ; we know 

 but little of the oceans as com- 

 pared with the land, and of the 

 land but little of the less civi- 

 lised regions. Whatever be the 

 lines of latitude cho<:en they 

 must pa.ss over sea, or desert, or 



both ; and if the assumptions made as to the magnetic con- 

 diions of these regions are incorrect, it may be that the results 

 are due not to the physical existence of the currents, but to the 

 inaccuracy of the data to which the formulae were applied. 



It therefore becomes important to check such large scale 

 calculations by others which depend only on the comparatively 

 small areas which have been minutely studied. 



In 1895, Dr. Carlheim-Gyllenskiold applied the test for the 

 existence of vertical currents to Sweden, for which comparatively 

 nuinerous observations could be utilised. The conclusion at 

 which he arrived was that there was no evidence for the existence 

 of the currents, except in those parts of the country where 

 the data were so untrustworthy as to make any conclusion 

 valueless. 



In 1896 similar calculations were made for this country. Dr. 

 Thorpe and I have recently completed a magnetic survey of the 

 United Kingdom which is, I believe, the most complete of any 

 which has hitherto been made of an equally large area. All our 

 observations were made within a few years, and, therefore, the 

 corrections for secular change were comparatively unimportant. 



The survey was divided into two parts, in one of which we 

 depicted the magnetic state of the kingdom in 1886 ; while the 

 other part was devoted to a similar investigation for the epoch 

 1891. We were thus able to compare the results obtained at 

 two periods, separated by a few years only, and by their con- 

 cordance or disagreement to judge of the value of our conclusions. 

 As these appear to be of some importance with regard to the 

 question we are now discussing, I have recently repeated the 

 calculations in a somewhat different way, and have determined 

 the average value of the currents flowing through all the districts 



Fig. 



