176 



solstice, and a rctnrn trom west lo cast betwcen Ihe soulhern 

 and the nortbern sulstice, Ihc amplitude being about 5 minutes 

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

 be disposed to nulicipate, those months, in which the temperature 

 at the surf'ace of our planet, or of the subsoU, or of the atmo- 

 sphere (as far as we posscss the means of judging of the tem- 

 perature of the atmospherc) attains its maximutn and minimum. 

 Stations so diversely silualed would indeed present in these 

 respects thermic conditions of greal variety: whercas uniforrnity 

 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 annual Variation at the hour of which wc are 

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

 or horary Variation at Ihat hour does actually diäappear, 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 ön, the eartVs 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 ou its axis, by which each raeridian suc- 

 cessively passes through every angle of inclination to the sun in 

 the round of 24 hours.« ©abineon the annual and diurnal 

 variations, in tiem nocfe nic^t erfc^ienenen 2ten Söanbe ber Ob- 

 servations made at the magn. and meteorol. Observa- 

 lory at Toronto p. XVII— XX. SSergl. auc^ feine SJbl^anblung 

 on the annual Variation of the magnetic Declination 

 at different periodsoftheDay in t)en Philos. Tr ansäet, 

 for 1851 P. II. p. 635, uub bie Siuleituiig in bie Observ. made 

 at the Observa tory at llobarton Vol. I. p. XXXIV— XXXVI. 

 •' (@. 80.) Sabine on the means adopted for delcr- 

 mining the absolute values,secularchange and annual 

 Variation of the terrestrial magnetic Force, in ben 

 Phil. Transact. for 1850 P. I. p. 216. 9lu* in ©abine'ö (Jr^ 

 fijfnungörebe bet ^Serfammlung ju Söelfaft (Meeting of the 



