Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/193

Rh TERRESTRIAL MAGNETISM.] was found by him that the lunar action was considerably increased when the sun was above the horizon of the place. 127. We have pointed out ( 119) that, while there is a marked likeness in many respects between the diurnal variation of declina tion and that of atmospheric temperature, we have yet no long- period fluctuation of the diurnal range of temperature at all com parable in magnitude to the magnetic fluctuations. It does not, however, seem difficult to account for this difference if we imagine that the magnetic fluctuations take their origin in the upper atmo spheric regions, while the temperature fluctuations are due to the lower regions of the earth s atmosphere. For, as the sun increases in power from times of minimum to times of maximum sun-spot frequency, we may imagine that a continuously increasing amount of aqueous vapour will be taken into the earth s atmosphere. Now the experiments of Tyndall and others induce us to think that the air would under such circumstances become more and more opaque for certain rays of the sun, and thus a continuously decreas ing proportion of the sun s heat would be able to penetrate into the lower atmospheric regions. This latter influence would there fore operate to cloak, perhaps to a considerable extent, the effect of the sun s increasing power ; and this may very well be the reason why the temperature range at the earth s surface does not exhibit the same eleven-yearly inequality as the declination range. 128. There seems, however, reason to believe that if we go from long to short period inequalities there is a much greater similarity in the range of the magnetical and the meteorological changes (113). Tiie explanation seems tobe that in the short-period changes the sun has not time to alter sensibly the constitution of the atmo sphere, and hence the proportional increase of effect experienced in the upper atmospheric regions is more nearly the same as that experienced near the surface of the earth. 129. Magnetic Disturbances. There is strong evidence that the most important disturbances break out very nearly simultaneously at widely different parts of the earth, and that they even affect both hemispheres at the same time. Very little, however, is known about the modus opcrandi of the forces concerned in producing such disturbances. For instance, it is not known whether a disturbance permanently affects the magnetic state of the earth, e.g., whether one of the magnetic elements before a disturbance begins is sensibly different in value from what it is after the disturbance has ceased to exist. On the other hand we know (1) that disturbances break out on the very day when there are rapid changes taking place on the sun s surface ( 83) ; (2) that they generally begin by momentarily increasing the horizontal force, but that the type quickly changes, so that during most disturbances the horizontal force is diminished ( 86) ; (3) that large disturbances take place more particularly about the equinoxes, when, we have reason to believe, the horizontal force of the earth is at a minimum ( 77). May we not possibly conclude from these habits of action that at times of disturbance the earth is magnetically in a delicate state of equilibrium, perhaps having more magnetism than its surroundings would strictly warrant, and being therefore inclined to part with some, and that a sudden increase of solar activity, tending, as such changes probably do, at first to exalt the magnetism of the earth, nevertheless destroys its magnetic balance and gives it ultimately the opportunity of parting with some of its magnetism ? This can only be regarded as a speculation, inasmuch as we do not know whether or not a disturbance produces any permanent influence upon the magnetism of the earth. 130. Auroras and Earth Currents. There is no doubt that these phenomena denote electric currents in the upper regions of the atmosphere and in the moist conducting crust of the earth. The point in dispute is with respect to the origin of such currents. Some are inclined to regard auroras as peculiar manifestations of atmospherical electricity in high latitudes, while others imagine that such displays are rather of the nature of induced currents generated by small but abrupt changes taking place in the magnetism of the earth. The advocates of the first view do not deny that currents taking place somehowin the upper atmospheric regions will have their conditions modified, to some extent at least, by the inducing influence of magnetic changes. Nor will the advocates of the induction hypo thesis be disposed to deny the possibility or even the certainty that displays due to atmospherical electricity and not dissimilar to some kind of aurora take place in some region of the atmosphere. But the first party regard auroras rather as the cause than as the effect of mag netic changes, whereas the advocates of induction regard such displays rather as the effect than as the cause of changes somehow produced in the magnetism of the earth. And here it is desirable to remark that the advocates of the induction hypothesis take for granted the magnetism of the earth and the changes thereof as phenomena for which they do not profess to account, whereas unless we go to some absolutely unknown cause (and this is against our present pro gramme) we must look to atmospherical electricity as likely to throw light upon the origin of terrestrial magnetism. We cannot therefore dispense with regarding atmospherical electricity as an agent which may have played an important part in the development of the present magnetical condition of the earth, but we are yet of opinion 183 that, under the present state of things, the theory which holds by atmospheric electricity must largely be supplemented by the induc tion hypothesis if it is to explain the peculiarities in type or form of the phenomena which observation brings before us. 131. Professor Tait in his essay on thunderstorms attributes one kind of aurora to atmospherical electricity. Such an aurora is, he believes, the manifestation of almost continuous discharges, like those given by a Holtz machine in a vacuum tube. The cause is condensation of vapour going on very slowly in very large spaces of air. The electricity is due to previous contact of particles of air and vapour. The result is that the air-particles in the mixture in time acquire a definite difference of potential from those of vapour, so that, when the latter aggregate, a misty region well charged is the result, and this discharges to the oppositely electrified air all round. 132. Again, Professor Stokes, without attempting to account for the origin of atmospherical electricity, has produced an hypothesis with the view of explaining the intimate connexion subsisting between auroral displays, earth currents, and magnetic changes on the one hand and outbursts of sun-spot activity on the other. His idea is that two somewhat distant atmospheric regions A and B are charged, let us say, with positive and negative electricity respec tively ; A induces in the ground below it a charge of negative, B a charge of positive electricity. At first things are held in this state : A cannot discharge either through the upper atmospheric regions to B or through the lower regions to the ground beneath it, while B is in a position precisely similar. Presently, however, an increase of the radiative power of the sun is produced. Such an increase would probably imply not merely an increase in general radiation but a particular increase in such actinic rays as are absorbed in the upper regions of the earth s atmosphere. The layer of atmosphere between A and B will therefore greedily absorb such rays, its tern- perature will rise, and, as is known to be the case for gases, the electrical conductivity of the stratum will be increased. A dis charge will therefore ultimately take place in the upper regions between A and B ; this will relieve the charges of negative and positive in the ground immediately beneath A and B, and these charges will therefore rush together through the ground, producing an earth current. This earth current will be in the opposite direc tion from the atmospheric current, and the two will combine to represent, virtually at least, if not absolutely, a closed circuit. This will of course affect the earth s magnetism and produce a disturbance. 133. This hypothesis certainly affords a good explanation of the promptness with which disturbances follow increased solar activity ( 83). Unless we are to resort to some unknown cause it is diffi cult to think of any other possible explanation of this fact. Such an explanation appears too to receive corroboration from the fact ( 97) that the lunar influence on the earth s magnetism as observed at Trevandrum is greater during the day than during the night, greater possibly too at times of maximum than at times of minimum sun-spots. We are therefore disposed to accept this explanation of the way in which increased solar activity produces magnetic dis turbance as the best that has been brought forward. 134. This does not, however, decide the disputed point how far these elevated currents are due to atmospherical electricity and how far to induction. The argument against the possibility of induced currents in these regions is derived from experiments with vacuum tubes, such as those recorded by Messrs De la Rue and Miiller, which would seem to indicate that enormous differences of potential would be required to produce electrical currents in elevated regions, where the atmosphere is very rare. Indeed, on account of these experiments, the measurements of the old observers, who sometimes assigned a height of more than 100 miles to the aurora, have been called in question, and it has been supposed against direct observation that these phenomena must always occur in regions much less elevated. It would appear too that such reasons were influential in determining Professor Stokes to regard the aurora as produced by atmospherical electricity which, as we know from ordinary lightning, presents us with enormous differences of potential ; but it is to be remarked that he has care fully guarded himself against the possibility of laboratory experi ments with vacuum tubes not being strictly analogous to that which takes place in the upper atmospheric regions. Now it would appeal- that recent experiments by Hittorf throw some doubt upon the strictness of this analogy. The high difference of potential required to force the current through vacuum tubes is, according to this observer, due in great part if not entirely to the passage of the fluid from the terminal to the residual air of the tube, so that the poten tial requisite to pass a current through a tube of double length is not sensibly greater than that required for a tube of single length. The whole subject is one which demands further investigation ; meanwhile we are not disposed to assert the impossibility of induc tion currents taking place in the upper atmospheric regions. 135. Let us now consider whether the form or type of the earth currents observed during disturbances favours the presence of induc tion to any sensible extent. The remarks of Dr Lloyd already quoted ( 93), which are confirmed by the Greenwich observations, seem to be decisive in this respect. These may be interpreted in