176 



solstice, and a retiirn froni west lo east between the southern 

 and the northern solstice, the amplitude being about 5 minules 

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

 be disposed lo anticipate, those months, in which the temperature 

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

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

 perature of ihe alniosphere) attains its maximum and minimum. 

 Stations so diversely situaled 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 similarity of character and 

 numerica! value. At all the stations Ihe 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 ihe diurnal 

 or horary Variation at thal hour does aclually disappear, are at 

 the equinoxesy when the sun is passing from the one hemisphere 

 to the olher, and when the magnetic direction in the course of 

 its annual Variation from east lo west, or vice versa, coincides 

 with the direction which is the raean declination of all the 

 months and of all the hours. — The annual Variation is ob- 

 viously connected wilh, and dependent on, the earth's position 

 in ils orbit relatively to the sun, around which it revolves; as 

 the diurnal Variation is connected wilh and dependent on the 

 rotation of the earth on its axis, by which each meridian suc- 

 cessivety passes through every angle of inclination to the sun in 

 the round of 24 hours.« ©abi neon the annual and diurnal 

 variations, in bem noct) nic()t errd)ieiteueii 2teit SÖanbe ber Ob- 

 servations made at the magn. and meteorol. übserva- 

 tory at Toronto p. XVll— XX. asergl. auc^ feine 9lbl)aubluii9 

 on the annual Variation of the magnetic Declination 

 at different periodsoftheDay in ben Philo s. Transacl. 

 for 1851 P. II. p. 635, unb bie (Sinleitung in tili Observ. made 

 at the Observatory at Ilobarton Vol. I. p. XXXIV— XXXVl. 

 *' (@. 80.) (Sabine on the means adopted fordeler- 

 mining the absolute values, secular change and annual 

 variition of the terrestrial magnetic Force, i\\ \>i\\ 

 Phil. Transact. for 1850 P. I. p. 21 G. 3Incl} in ©abine'ö Sr- 

 cffnungörebe ber ^Iserfammliinö ju 93elfaft (Meeting of the 



