MAGNETIC VAKIATION. 123 



minimum at 12 P.M. or 2 A.M.; and the second maximum at 

 2 A.M. or 4 A.M. I must here content myself with merely 

 giving the mean conditions, drawing attention to the fact, 



Makerstoun. Toronto. Washington. 



1842-43. 1845-47. 1840-42. 



Maximum Oh. 40m. 1 P.M. 2 P.M. 



Minimum 10 P.M. 10 P.M. 10 P.M. 



Maximum 2h. 15m. A.M. 2 A.M. 2 A.M. 



Minimum 7h.l5m.A.M. 8 A.M. 8 A.M. 



The different seasons exhibited some striking differences at Greenwich. 

 In the year 1847 there was only one maximum (2 P.M.) and one mini- 

 mum (12 night) during the winter; in the summer there was a double 

 progression, but the secondary minimum occurred at 2 A.M. instead of 

 4 A.M. (p. 236). The greatest western elongation (principal maximum) 

 remained stationary at 2 P.M. in winter as well as in summer, but the 

 smaller or secondary minimum fell, in 1846, as usual (p. 94), at about 

 8 A.M. in the summer, and in winter about 1 2 at night. The mean whiter 

 western elongation continued without intermission throughout the whole 

 year between midnight and 2 P.M. (see also for 1845, p. 5). We owe the 

 erection of the observatory at Makerstoun, Roxburghshire, in Scotland, 

 to the generous scientific zeal of Sir Thomas Brisbane (see John Allan 

 Broun, Obs. in Magnetism and Meteorology made at Makerstoun in 1843, 

 pp. 221 227). On the horary diurnal and nocturnal observations of 

 St. Petersburg, see Kupffer, Compte-rendu Meteor, et Mag. a Mr. de 

 Brock en 1851, p. 17. Sabine, in his admirable and ingeniously com- 

 bined graphic representation of the curve of horary declination at 

 Toronto (Phil. Transact, for 1851, pt. ii, plate 27), shows that there is 

 a singular period of rest (from 9 to 11 P.M.) occurring before the small 

 nocturnal western motion, which begins about 11 P.M., and continues 

 till about 3 A.M. " We find," he observes, " alternate progression and 

 retrogression at Toronto twice in the 24 hours. In 2 of the 8 quarters 

 (1841 and 1842) the inferior degree of regularity during the night occa- 

 sions the occurrence of a triple max. and min.; in the remaining quar- 

 ters the turning hours are the same as those of the mean of the 2 years." 

 (Obs. made at the Magn. and Meteor. Observatory at Toronto, in Canada, 

 vol. i, pp. xiv, xxiv, 183 191, and 228; and Unusual Magn. Distur- 

 bances, pt. i, p. vi.) For the very complete observations made at Wasb- 

 ington, see Gilliss, Magn. and Meteor. Observations made at Washington, 

 p. 325 (General .Law). Compare with these Bache, Observ. at the Magn. 

 and Meteor. Observatory at the Girard College, Philadelphia, made in the 

 years 1840 to 1845 (3 volumes, containing 3212 quarto pages) vol. i, 

 p. 709, vol. ii, p. 1285, vol. iii, pp. 2167, 2702. Notwithstanding the 

 vicinity of these two places (Philadelphia lying only 1 4' north, and 

 0* 7' 33" east of Washington), I find a difference in the lesser periods 

 of the western secondary maximum and secondary minimum. The 

 former falls about Ih. 30m. and the latter about 2h. 15m. earlier at 

 Philadelphia. 



