MAGNETIC OBSERVATIONS. 101 



secondary maximum at 8 A.M., and a weaker secondary 

 minimum at 10 A.M. The intensity at Hobarton, on 

 the contrary, exhibits a simple progression from a maxi- 

 mum between 5 and 6 P.M. to a minimum between 8 

 and 9 A.M. ; although the inclination there, no less than 

 at Toronto, exhibits four turning points ao . By a com- 

 parison of the variations of inclination, with those of the 

 horizontal force, it has been established that, in Canada, 

 during the winter months, when the sun is in the southern 

 signs of the zodiac, the total terrestrial force has a greater 

 intensity than in the summer months, whilst in Van 

 Diemen's Land the intensity is greater than the mean annual 

 value that is to say, the total terrestrial force from 

 October to February, which constitutes the summer of the 

 southern hemisphere, while it is less from April to August. 

 According to Sabine, 21 this intensity of the terrestrial mag- 

 netic force is not dependent on differences of temperature, 

 but on the lesser distance of the magnetic solar body from 

 the earth. At Hobarton, the intensity during the summer 

 is 13.574 in the absolute scale, whilst during the winter it fe 

 13.543. The secular variation of intensity has hitherto been 

 deduced from only a small number of observations. At 

 Toronto, it appears to have suffered some decrease between 

 1845 and 1849, and the comparison of my own observations 

 with those of Rudberg, in the years 1806 and 1832, give a 

 similar result for Berlin. 22 



20 Sabine, in Magn. and Meteor. Observations at Hobarton, vol. i, 

 p. Ixviii. " There is also a correspondence in the range and turning 

 hours of the diurnal variation of the total force at Hobarton and at 

 Toronto, although the progression is a double one at Toronto and a 

 single one at Hobarton." The time of the maximum of intensity falls 

 at Hobarton between 8 and 9 A.M.; whilst the secondary or lesser mini- 

 mum falls at Toronto about 10 A.M., and consequently the increase and 

 diminution of the intensity fall within the same hours in accordance 

 with the time of the place, and not at opposite hours, as is the case 

 with respect to the inclination and the declination. See, regarding the 

 causes of this phenomenon, p. Ixix (compare also Faraday, Atmospheric 

 Magnetism, 30273034). 



21 Phil. Transact, for 1850, pt. i, pp. 215217; Magnet. Observ. 

 at Hobarton, vol. ii, 1852, p. xlvi. See also p. 22 of the present 

 volume. At the Cape of Good Hope the intensity presents less differ- 

 ence at opposite periods of the year than the inclination (Magnet. 

 Observ. made at the Cape of Good Hope, vol. i, 1851, p. lv). 



22 See the magnetic part of my work on Asie Centrale, t. iii, p. 442. 



