MAGNETIC INTENSITY. 83 



the maxima and minima of the temperature of the atmo- 

 sphere, or of the upper crust of the earth. We may remark 

 that the annual alterations were first accurately represented 

 by Sabine from a very large number of observations. The 

 turning points in the most important magnetic phenomena 

 are the solstices and the equinoxes. The epoch at which 

 the intensity of the terrestrial force is the greatest, and that 

 at which the dipping needle most nearly assumes the vertical 

 position in both hemispheres, is identical with the period at 

 which the earth is nearest to the sun, 81 and consequently 

 when its velocity of translation is the greatest. At this 

 period, however, when the earth is nearest to the sun, namely 

 in December, January, and February ; as well as in May, 

 June,, and July, when it is farthest from the sun, the relations 

 of temperature of the zones on either side of the equator are 

 completely reversed, the turning points of the decreasing and 



the annual variation at the hour of which we are treating. The only 

 periods of the year in which the diurnal or horary variation at that 

 hour does actually disappear are at the equinoxes, when the sun is 

 passing from the one hemisphere to the other, and when the magnetic 

 direction, in the course of its annual variation from east to west, or 

 vice vers;\, coincides with the direction which is the mean declination 

 of all the months and of all the hours. The annual variation is obvi- 

 ously connected with, and dependent on, the earth's position in its orbit 

 relatively to the sun around which it revolves; as the diurnal variation 

 is connected with, and dependent on, the rotation of the earth on its axis, 

 by which each meridian successively passes through every angle of in- 

 clination to the sun in the round of 24 hours." Sabine, on the Annual 

 and Diurnal Variations, in the second volume of Observations made at 

 the Magn. and Meteor ol. Observatory at Toronto, p. xvii xx. See also 

 his memoir, On the Annual Variation of the Magnetic Declination at 

 different periods of the day, in the Philos. Transact, for 1851, pt. f 

 p. 635, and the Introduction of his Observ. made at the Observatory a 

 Hobarton, vol. i, p. xxxiv xxxvi. 



81 Sabine, On the means adopted for determining the Absolute Values, 

 Secular Change, and Annual Variation of the Terrestrial Magnetic Force, 

 in the Phil. Transact, for 1850, pt. i, p. 216. In his address to the Asso- 

 ciation at Belfast (Meeting of the Brit. Assoc. in 1852), he likewise 

 observes, " that it is a remarkable fact which has been established that 

 the magnetic force is greater in both the northern and southern hemi- 

 spheres in the months of December, January, and February, when the 

 sun is nearest to the earth, than in those of May, June, and July, when 

 he is most distant from it : whereas, if the effects were due to tempera- 

 ture, the two hemispheres should be oppositely, instead of similarly, 

 affected in eacli of the two periods referred to." 



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