176 



solstice, aud a retiirn trom wesl lo east between Ihe southern 

 and the northern solstice, ihe ampütude being about 5 minutes 

 of arc. The turning periods of the year are not, as many raight 

 be disposed to anticipate, those months^ in which the tetnperature 

 at the surf'ace of our planet . or of the suhsoil ^ or of the atmo- 

 sphere (as far as wo possess the means of judging of the tem- 

 perature of the atmosphere) attains its maximnm and minimum. 

 Stations so diversely situated would indeed present in these 

 respects thermic conditioiis of great variety: whereas uniforniity 

 in the epoch of the turning periods is a not less conspicuous 

 feature in the annual Variation than similarity of character and 

 numerical value. At all the stations the solstices are the turning 

 periods of the annuai Variation at the hour of which we are 

 Ireating. — The oniy periods of the year in which the diurnal 

 or horary Variation at that hoar 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 ob- 

 viously 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 suc- 

 cessively passes through every angle ofinclination to the sun in 

 the round of 24 hours. a ©abtue on the annual and diurnal 

 variations, \\\ bem nod) ntd)t erfdjieneneu 2teii Söanbe ber Ob- 

 servations raade at the magn. and meteorol. Observa- 

 tory at Toronto p. XVII— XX. QSeröl. aiid) feine 2lbl)anblun9 

 on the annual Variation of the magnetic Declination 

 at different periodsoftheDay in beii Philos. Tr ansäet, 

 for 1851 P. li. p. 635, iinb bie (Jinleituuö tu tiU Observ. made 

 at the Observatory atHobarton Vol. I. p. XXXiV'— XXXVI. 

 *' (@. 80.) Siabiue on the means adopted fordeter- 

 mining the absolute values, secularchange and annaal 

 Variation of the terreslrial magnetic Force, in ben 

 Phil. Transact. for 1850 P. 1. p. 216. '}i\\&i in ©abine'ö €r- 

 Sjfnungörebe ber ^erfammlung ju 33elfa|l: (Meeting of the 



