Page:The American Cyclopædia (1879) Volume XI.djvu/19

 TERRESTRIAL MAGNETISM MAGNETO-ELECTRICITY 11 the W. side in the afternoon, corresponding variations in the needle will be exhibited. In the other half of the year, for a similar reason, the S. end of the needle will be affected in an analogous but opposite manner ; the strength of the magnetism of the earth will be increased by the nearer approach of the sun, in the same way that two magnets having their dissimilar poles opposite each other are increased or diminished in magnetic power by a diminu- tion or decrease of distance. We are indebted for the interesting discovery of the polar ac- tion of the sun to Gen. Sabine of England, who has had charge of the reduction of all the magnetic observations of the English colo- nial observatories ; and to Dr. Kriel of Aus- tria for another of the same character, which leads us to extend the principle of magnetism to the moon. It is found that there is a varia- tion of each of the magnetic elements corre- sponding with the diurnal position of the moon in regard to the earth ; but this resembles the tides in exhibiting two maxima and two mini- ma in the course of 24 hours, regularly chang- ing in time with the motion of the moon in her orbit around the earth. These phenomena in- dicate that the moon is not magnetic per se, that is, possessed of permanent magnetism, but its magnetic condition resembles that of soft iron developed by the continued but varying inductive influence on account of change of distance of the earth and the sun. That these changes in the magnetic elements cannot be due to heat in this case, must be evident, since the temperature of the moon as a mass is but little greater than that of celestial space. The third class of variations, which was formerly denominated fitful, is now known in a cer- tain sense to be periodical. They were called by Hnmboldt magnetic storms, and were found by Arago to accompany the appearance of the aurora borealis. Although it is impos- sible to predict from our present knowledge the recurrence of individual cases of these great perturbations in the intensity and direction of the magnetism of the earth, yet they are known to increase in number and magnitude of ac- tion within the period of a little more than five years, and gradually to diminish .through nearly an equal period, the whole cycle being completed in a little more than 11 years. The magnetic storms have been observed in the most distant parts of the earth, and no doubt can now exist as to their cosmical character. The lunar influence of which we have just spoken does not appear to participate in or be connected with this inequality. The period- icity of these apparently fitful variations of magnetism was first pointed out by Gen. Sa- bine, and has since been established by the in- vestigations of Prof. Lloyd of Ireland, Dr. La- ment of Germany, and by those of Prof. Bache from the observations made under his direc- tion at Girard college. But the most astonish- ing result in regard to this class of perturba- tions is that they coincide with the periodical recurrence of the maxima and minima of the spots on the sun. A German astronomer, Schwabe, has established, by nearly 30 years of unremitting daily observation, the periodicity of this phenomenon. He finds that the solar spots increase in magnitude for about 5 years, and diminish through an equal period, the cy- cle, as in the case of magnetic storms, being completed in about 11 years. The discovery of a connection of this remarkable kind gives to magnetism a high position in the scale of distinct natural forces, and assigns to it equally with gravitation a truly cosmical character. It is not impossible that the spots on the sun may be connected with the falling into its gaseous envelope of meteorites, and this suggestion is favored by an observation of Mr. Carrington of England, in which a remarkable appear- ance was observed on the surface of the sun, analogous to that which would have been pro- duced by an occurrence of the kind we have mentioned. Recently Prof. Loomis of Yale college has published his analysis of the obser- vations of many past years, apparently placing beyond all question the existence of a connec- tion between the sun-spot period, terrestrial magnetic disturbances, and the frequency of auroras. One of the most interesting ques- tions belonging to the future of this subject, is the possible existence of an association be- tween the phenomena of the sun's colored prominences and the magnetic activity of the earth. Observations by Prof. Young of Dart- mouth college seem to show the extreme prob- ability of such an association. Moreover, the observations which have been made on the prominences, by showing a connection between these objects and the solar spots, seem to force upon us the conclusion that some relation ex- ists between the colored flames and the phe- nomena of terrestrial magnetism, since the partial dependence of these upon the sun's con- dition as to spots has been very nearly if not quite demonstrated. It is not intended by what has been said to convey the idea that meteorological changes may not affect the po- sition of the needle, and that even the magnet- ic condition of the atmosphere, according to the hypothesis of Faraday, may not produce appreciable results ; but as yet the actions of these appear to neutralize each other, and to leave no definite record of their existence in the course of periods of considerable length. It is probable, however, that with the im- proved photometrical instruments and a more minute scrutiny of their records, the effects due to these causes will be shown. Since the agitation of the atoms of an iron bar is found to favor the development of magnetism by in- duction, it is not improbable that the magnet- ism of the earth may be disturbed during the continuance and shortly after the occurrence of an earthquake. MAGNETO-ELECTRICITY. As shown in the article ELECTKO-MAGNETISM, great magnetic power is developed by passing a current of