XX11 NOTES. 



east between the southern and the northern solstice, the amplitude being about 

 five minutes of arc. The turning periods of the year are not, as many might 

 be disposed to anticipate, those months in which the temperature at the surface 

 of our planet, or of the subsoil, or of the atmosphere (as far as we possess the 

 means of judging of the temperature of the atmosphere) attains its maximum 

 and minimum. Stations so diversely situated would indeed present in these 

 respects thermic conditions of great variety: whereas uniformity in the epoch of 

 the turning periods is a not less conspicuous feature in the annual variation than 

 is similarity of character and numerical value. At all the stations the solstices 

 are the turning periods of 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 versa, coincides 

 with the direction which is the mean declination of all the months and of all the 

 hours. The annual variation is obviously 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 inclination to the sun in the round of twenty-four hours." Sabine, On 

 the annual and diurnal Variations (Observations made at the Magn. and Meteonol. 

 Observatory at Toronto, vol. ii. p. xvii xx.). Compare also his Memoir on the 

 annual Variation of the magnetic Declination at different Periods of the Day, in 

 the Phil. Trans, for 1851, Pt. II. p. 635, and the Introduction to vol. i. of the 

 Observations made at the Observatory at Hobarton, p. xxxiv xxxvi. 



( 81 ) p. 88. Sabine, On the Means adopted for determining the absolute Values, 

 secular Change, and annual Variation of the terrestrial magnetic Force, in the 

 Phil. Trans, for 1850, Pt. I. p. 216. Also in Sabine's Opening Address to the 

 Meeting of the British Association at Belfast, in 1852, he says: " It is a re- 

 markable fact, which has been established, that the magnetic force is greater in 

 both the northern and southern hemispheres 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 

 temperature, the two hemispheres should be oppositely instead of similarly 

 affected in each of the two periods referred to. 



( 82 ) p. 88. Lamont, in Poggend. Ann. Bd. 84, S. 579. 



(") p. 89. Sabine, On periodical Laws discoverable in the mean Effects of the 

 larger magnetic Disturbances, in the Phil. Trans, for 1852, Pt.I. p. 12 1. (Kos- 

 mos, Bd. iv. S. 73; English edition, p. 78.) 



