Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/191

Rh TERRESTRIAL MAGNETISM.] Scientifically we may regard the earth as an engine, of which the sun is the furnace, the equatorial regions the boiler, and the polar regions the condenser. Now this engine may be supposed to work in the following manner. Hot air and vapour are carried along the upper regions of the atmosphere from the equator to the poles by means of the anti-trade winds, while in return the cold polar air is carried along the surface of the earth from the poles to the equator, forming what is known as the trade-winds. When ever the sun s heat is most powerful both trades and anti-trades should be most powerful likewise. But we live in the trades rather than in the anti-trades in the surface currents, and not in the upper currents of the earth s atmosphere. When, therefore, the sun is most powerful is it not possible that we might have a parti cularly strong and cold polar current blowing about us ? The same thing would happen in the cr.se of a furnace-tire ; the stronger the tire the more powerful the hot draught up the chimney, the more powerful also the cold draught from without along the tloor of the room. It might thus follow that a man standing in the furnace- room near the door might be chilled rather than heated when the furnace itself was roaring loudest. In fact temperature is a pheno menon due to many causes. Thus a low temperature may be due (1) to a deficiency in solar power, (2) to a clouded sky, (3) to cold rain, (4) to cold winds, (5) to cold water and ice, (6) to cold pro duced by evaporation, (7) to cold produced by radiation into space. Blanford has recently slibwn that at certain Indian stations a low mean temperature occurs when there is an unusually large rainfall and a great amount of clouds, a result in accordance with the conclusions previously enunciated by Professor Piazzi Smyth. Records of maximum and minimum temperature must not there fore be too closely associated with a maximum and minimum of solar power. 113. Considerations of this nature have induced Stewart to imagine (Nature, June 16, 1831) that the true connexion between sun-spots and terrestrial temperature is more likely to be discovered by a study of short-period inequalities of sun-spots than by that of the eleven-year period in which there is time enough to change the hygrometric state of tha atmosphere and the whole convection system of the earth. He has accordingly discussed at some length two prominent sun-spot inequalities of short period (about twenty- four days), and endeavoured to see in what way they affect terres trial temperature. From this it appears that a rapid increase of sun-spots is followed in a day or two by an increase of the diurnal temperature range at Toronto. Now an increase of diurnal tempera ture range most probably denotes an increase of solar energy, and we are thus led to associate an increase of solar heat with a large development of spots. This, however, is a point which requires further investigation. 114. General Conclusion. On the whole &quot;we may conclude that the meteorological motions and processes of the earth are probably most active at times of maximum sun-spots, and that they agree with magnetical phenomena in representing the sun as most power ful on such occasions, although the evidence derived from meteor ology is not so conclusive as that derived from magnetism. HYPOTHETICAL VIEWS REGARDING THE CONNEXION BETWEEN THE STATE OF THE SUN AND TERRESTRIAL MAGNETISM. 115. Principles of Discussion. In the following discussion we claim only to advance a working hypothesis, with the view of sug gesting further inquiries into the subject of terrestrial magnetism. It seems therefore desirable that we should limit ourselves to such probable or possible causes as are known to exist and to operate on the earth. These various agents or causes will be described, and we shall endeavour to show that converging lines of evidence point in several cases to certain of these as being most likely to produce that particular type of effect which is exhibited in terrestrial magnetism. This course will in our view most readily suggest further inquiries with the view of confirming or disproving the various points of this working hypothesis. Believing that the introduction of any unknown cause can only be justified when known causes have been found insufficient to account for the pheno mena in question, we have not advocated any direct magnetic action of the sun upon the earth. We have refrained from this for two reasons, first, because from what we now know of the sun it appears to us unlikely that it should exercise an influence of this nature upon the earth, since a body at a high temperature possess ing very strong magnetic properties is unknown to us ; and, secondly, we shall see further on that such an influence will not explain the best-understood magnetic changes, nor is there in our opinion any magnetic phenomenon for the explanation of which it appears absolutely necessary to resort to this hypothesis. In fine, without presuming to deny the possibility of unknown influences of this naiure, It does not appear to us that the time has yet arrived when we are called upon to resort to such as necessary aids to the discovery of further truth. 116. Nature of Solar Variations. It is quite certain that there is a variability in the visible appearance of the sun s disk, which exhibits sometimes a comparatively large amount of spotted area 181 while on other occasions it is entirely free from spots. Now it has been remarked by Thomson that were the sun an incandescent solid its surface would become cool in a few minutes. The astonishing property which our luminary possesses of pouring out continuously a vast amount of radiant energy must unquestionably depend upon machinery of great power by means of which fresh hot particles are rapidly brought from the interior to the surface, while those particles which have given out their light and heat are rapidly hurried downwards to be recruited from the great storehouse of heat in the sun s centre. In fine, a gigantic system of convection currents of this nature forms the essential condition without which the sun would not be able to continue shining as it does. The mottled appearance of the sun s disk as seen through a tele scope denotes no doubt the existence of a vast system of ascending and descending currents, the hot matter rising from beneath being denoted by the blighter portions and the cold matter descending from above by the darker portions of the structure. On certain occasions and in certain regions of the sun the scale of these pheno mena is greatly increased, and we have a huge up-rush of bright and a corresponding down-rush of black matter in fine, the well- known sun-spot with its bright faculous appendages. Whenever sun-spots are very frequent we should therefore expect the convection system of the sun to be particularly powerful, and the great velocity and size of the red flames or the higher portions of the convection system observed around the sun s limb on such occasions confirm us in this supposition. And if the convection system of the sun be particularly powerful when there are most spots on its surface, it would seem to follow that the radiation from our luminary should on such occasions be particularly powerful also. The spectroscope leads us to the same conclusion. It would appear from the obser vations of Lockyer and others that at times of maximum sun-spots certain definite regions of the sun when examined spectroscopically present all the appearances of a very high temperature. We are unable to confirm these conclusions by direct observations of the sun s heating power. Actinometrical determinations have not yet been made with sufficient accuracy and persistence to decide this point experimentally. We have, however, evidence of an in direct nature derived from terrestrial magnetism and meteorology all tending to make us think that the sun is most powerful during times of maximum sun-spots. We have seen that on such occasions the solar influence npon the magnetism of the earth is peculiarly powerful in more than one way, and that its influence on meteoro logy is then peculiarly powerful also, although we are not so certain of this latter fact as of the former. We may therefore take it to be most probable that the sun is most powerful at times of maximum sun-spots, and proceed from this basis to propound the two following questions : in the first place, what is the nature of the solar influence upon terrestrial magnetism ? and, secondly, why is this influence so much more easily perceived than certain forms of solar influence upon meteorology ? 117. Diurnal Magnetic Variations Hypotheses regarding them. The various speculators on the cause of these phenomena have ranged over the whole field of likely explanations. (1) It has been supposed that the sun acts directly as a magnet npon the magnetism of the earth. (2) It has been imagined that convection currents established by the sun s heating influence in the upper regions of the atmosphere are to be regarded as conductors moving across lines of magnetic force, and are thus the vehicle of electric currents which act upon the magnet. (3) Faraday, reasoning from his discovery that oxygen is paramagnetic, and becomes weaker in its power when heated, and stronger when cooled, supposed that the sun by heating certain portions of the atmosphere renders them less magnetic while others, not subjected to any heating influence are rendered more magnetic. The action is equivalent to a dis placement by means of the sun of the magnetic matter of the earth, and involves a displacement of the lines of force. Here too the solar action is associated with the great mass of the atmosphere. (4) It has been supposed by Christie and by De la liive that the heat of the sun produces in the atmosphere and in the earth thermo electric currents which produce the daily magnetic variations. It is not easy to perceive how we could have thermo-electric currents in the upper regions of the atmosphere, but there is no obvious objection to the generation of such currents in the crust of the earth. Thus the first hypothesis has no reference to the atmosphere what ever ; the second deals with the upper atmospheric regions, the third with the great body of the atmosphere, while the fourth, as we have ventured to modify it, has reference to the crust of the earth. 118. Discussion of these Hypotlicses. Dr Lloyd and Mr C. Chambers 1 have both shown that direct solar magnetic action will not account for the peculiarities of the diurnal magnetic variation. Again ( 48) we have strong evidence that changes in the range of the daily magnetic variation lag behind corresponding solar changes in point of time. Now this kind of behaviour is inconsistent with direct magnetic action, and points rather to an indirect magnetic eft ect caused by the radiant energy of the sun. 1 Proc. Royal Irish Academy, February 22, 1S58; Phil. Iran. ., 1SCO.