Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/192

Rh 182 METEOROLOGY [TERRESTRIAL MAGNETISM. 119. Let us therefore dismiss the hypothesis of direct action and consider that of Faraday. We know both from observations of the declination and horizontal force (Proc. Roy. Soc., March 22, 1877, and Phil. Trans., 1880, p. 541) that the action of the sun in pro ducing diurnal variations of these elements is one and a half times as powerful at epochs of maximum as it is at epochs of minimum sun-spot frequency. It is hardly credible that there should be such a great difference on these occasions in the sun's heating effect upon the great bulk of the atmosphere. Meteorologists have never observed such a difference, nor is there any marked corresponding inequality of diurnal temperature range. Meteorological evidence is thus against the diurnal magnetic changes being due to the heating up by the sun of the great mass of oxygen which constitutes the magnetic portion of the earth's atmosphere. Again, as there is a preponderance of hot oxygen in the northern hemisphere during the June and in the southern hemisphere during the December solstice, there ought according to this theory to be a well-marked annual variation of the magnetism of the earth, the northern hemi sphere being at the same time differently affected from the southern. But there are no traces of such a phenomenon, the annual and semi annual variations which we have already described (§§ 64-67) being of quite a different nature, and none of them very large.

120. Precisely the same objections apply with even greater force to the fourth hypothesis. It seems impossible to allow that any heating effect of the crust of the earth caused by the sun can be one and a half times as great at epochs of maximum as it is at epochs of minimum sun-spot frequency.

121. We are thus driven by the method of exhaustions to look to the upper regions of the earth's atmosphere as the most probable seat of the solar influence in producing diurnal magnetic changes, and it need hardly be said that the only conceivable magnetic cause capable of operating in such regions must be an electric current. Now we know from our study of the aurora that there are such currents in these regions continuous near the pole and occasional in lower latitudes. A good deal has been said about the difficulty of imagining a daily set of currents to be generated in regions of such imperfect conductivity, but we shall see by and by ( 134) that there seems ground for imagining that their conductivity may be much greater than has hitherto been supposed.

122. Analogies between the Meteorological and Magnetical Systems of the Earth. We have in the first place a zone of maximum ter restrial temperature, the middle line of which is nearly coincident not only with the geographical but likewise with the magnetical equator. Again, there are possibly in the northern hemisphere two poles of greatest cold, which possibly do not greatly differ in position from those spots which we have called magnetic poles or foci. About the southern hemisphere we have no information.

Furthermore we believe that the hot air is carried from the zone of greatest heat to the place or places of greatest cold by means, no doubt, of the return trades which blow in the upper atmospheric regions. The hot air divides at this zone, one part blowing north wards in the northern and another southwards in the southern hemisphere. Now this zone, from which the anti-trades divide, has an annual motion of its own, being found farthest north at the June solstice and farthest south at that of December. Probably too the northern system is strongest in June and the southern system in December. If we now turn to the solar-diurnal variation of magnetic declination, we find here also a northern and a southern system (§ 41), the type of the one being antagonistic to that of the other. We find also that the northern system is strongest in June and the southern system in December.

Again, it seems probable from what we have now said that the anti-trades, strictly speaking, have reference not to the geographical equator and poles but to the zone of maximum and the poles of minimum temperature. Now, turning once more to the diurnal oscillations of the declination needle, it seems probable that the directions east and west must be interpreted as having a reference not to the geographical but to the magnetical pole (§ 45).

These analogies must be taken for what they are worth. Our object in introducing them has reference to the previous discussion, from which we concluded that the magnetic influence of the sun is probably due to currents in the upper region of the atmosphere the cause of which we were content to leave in abeyance. Now these analogies would lead us to suggest that this cause, whatever it is, may perhaps be found to be related to the convection system of the earth on the one hand and to the magnetic system on the other.

123. Analogies between Meteorological and Magnetical Weather. These remarks are borne out by the further analogy which appears to subsist betwixt what we have termed meteorological and mag netical weather. Let us take the solar-diurnal variation of declina tion. Not only is this variation similar in form to the diurnal variation of atmospheric temperature ( 37), but the ranges of the tvo have a similar annual variation. And, as the element of meteorological weather affects the orderly march of the temperature range, just so the element of magnetical weather affects the orderly march of the declination range.

Furthermore, just as temperature-range weather progresses from west to east (§ 52), so declination-range weather would seem to pro gress in the same direction as the other (§ 52) although at a greater rate. It will doubtless require a more extended investigation to make us quite sure of this latter point; nevertheless we do not perceive the validity of the objection that is sometimes made to the hypothesis of progress in magnetic weather on the ground that magnetic influences are known to affect all portions of the globe simultaneously. It will, we think, be perceived that in the above statement no supposition whatever is made with respect to the rate of propagation of a magnetic influence through the earth ; this may be instantaneous or it may not. It is supposed that we have here a travelling cause of excitement, say a travelling cause of currents in the upper regions of the atmosphere which progresses from west to east and always produces its most marked effect above those regions where it passes just as the sun itself in passing from east to west produces a magnetic effect the various phases of which travel from east to west with the sun which causes them. We think too that this hypothesis of travelling causes of magnetic change is strengthened by the facts observed by Capello and described in § 97.

124. If, however, the objection made to this hypothesis refers to the fact disclosed by Broun (§ 85) that changes of horizontal force appear to take place simultaneously at distant parts of the earth's surface, then we think that analogy should lead us not to deny the possibility of a travelling magnetic excitement, but rather to suggest the possibility of there being some meteorological influence which, like the magnetical one above mentioned, may be found to take place simultaneously at different parts of the earth's surface. Now Broun (Proc. Roy. Soc., May 11, 1876) has given us preliminary evidence for supposing that there are simultaneous barometric varia tions. For instance, there was a barometric maximum at Hobart Town, Peking, the Cape, St Helena, Makerstoun, Singapore, Madras, Simla, Ekaterinburg, and Bogoslovsk about the end of March or first day of April 1845. There appears to have been a simultaneous increase of the horizontal force of the earth at various stations much about the same time, and there also appears to have been a short-period maximum of spots on the solar surface. Broun has likewise registered simultaneous barometric variations at Singa pore, Madras, and Simla, for the first three months of 1845. From these it would seem that simultaneous barometric maxima are possibly coincident with rapidly increasing sun-spot areas. Again is it not absolutely certain that if there is a sudden increase of solar power this must mean an increase of heat communicated to the earth, although it may be difficult or even impossible to obtain experimental evidence of such a fact? All these are sub jects which require further investigation.

125. Further Remarks on the Solar-Diurnal Variation of Declination. In § 24 we have asked how far the action of the solar- diurnal force upon a freely-suspended magnet is due to currents acting directly upon the magnet and how far to a change produced in the magnetism of the earth. Some light appears to be thrown on this point by the behaviour of the needle at places near the magnetic pole where the dipping needle is nearly vertical. On opposite sides of this locality the declination needle points in oppo site directions. Now suppose that we have a set of such needles placed all round this region. It seems a legitimate generalization from the observations described by Sabine (§ 45) to conclude that if we place ourselves above the centre of any of these needles at 8 A.M., and look towards its marked pole, we shall find it in every case deflected towards the right, while if we look towards the same pole at 2 P. M. we shall find it deflected to the left. Now if we imagine that at 8 A.M. there are above these magnets (in the upper atmospheric regions) electrical currents of which the horizontal components form a set of positive currents flowing from the pole on all sides, then by the known laws of such currents the marked pole of all these needles will be deflected towards the right. And if at 2 P.M. the resolved portions of such currents should be flowing towards the pole, then the marked poles of all these needles will be deflected towards the left. It thus appears that this peculiar magnetic behaviour might easily be explained by a hypothetical distribution of currents. And in fact in such regions we have in dubitable evidence of the existence of currents in the upper regions of the atmosphere. On the other hand this behaviour could not easily be explained by the hypothesis of some definite temporary magnetic system set up by the solar Influence in the earth, for in such a case we should imagine that similar poles of all the needles ought to be deflected towards the pole of this temporary system, which is not the case.

126. Another point for consideration is the possible complexity of the solar-diurnal variation. For we may imagine (1) that the sun acts in such a manner as to produce a diurnal variation; (2) it may also act like the moon (§ 94) and produce a semidiurnal variation; (3) these possible actions may be accompanied by induced currents in the upper regions of the atmosphere and in the crust of the earth; (4) it is possible that the sun's rays may affect these variations or some of them in the way in which Broun found that the lunar variation at Trevandrum was affected by the sun. It