Total Magnetic Foece. 



395 



Secular Change of the Total Force. — This deduced from the secular changes for the two components, pages 356 

 and 374, = - 0001388. 



Annual Period of the Total Magnetic Force. — The following quantities have been obtained from the means, 

 pages 356 and 374, exhibiting the annual periods for the two components freed from secular change : — 



Jan. 



0-000 I 322 



Feb. 

 121 



ilarch. April. 



117 309 



Slay. 



212 



June. 



218 



July. 

 090 



Aug. 

 000 



Sept. 



053 



Oct. 

 048 



Nov. 

 233 



Dec. 

 266 



The accuracy of this result depends chiefly on that for the vertical component, which it resembles in every 

 respect. The total magnetic force is a minimum in August and a maximum in January or December ; it is also 

 a secondary minimum in the beginning of March and a maximum in April.* 



TABLE LVII. — Variations of the Total Magnetic Force, with reference to the Moon's Age, 

 Declination, and Distance, as deduced from Tables XXIII. and XXXIX. 



* Note on the Annual Variation of the Total Magnetic Force. — In the note, p. 357, it has been mentioned that the observations of 

 the bifilar magnetometer, made at Toronto in 1842, had been discussed, and that the same law had resulted as from the Makerstouu 

 observations. The observations of the bifilar magnetometer made at St Helena, in the four years, 1842-3-4-5, have been examined 

 in a similar manner. The temperature coefficient has been deduced from the bifilar observations, and the observations have been 

 corrected by it. The variations of the horizontal component at St Helena may be considered as the variations of the total magnetic 

 force, as much at least as they are the representatives of the varying magnetic inclination at Toronto and Makerstoun. The mean of 

 the results for the four years (three years only for the first ten weeks) indicates that the horizontal component at St Helena is a maxi- 

 mum in the beginning of June, and a minimum in the beginning of December : this result differs from that obtained by Colonel 

 Sabine from two years of the same observations (chiefly, perhaps, because the results for the separate years are not very consistent 

 among themselves.) In Colonel Sabine's result there appears to be a connection between the intensity and temperature of the air ; the 

 magnetic force appearing greatest when the temperature is greatest. Such a result would be in opposition to the well-known con- 

 nection between the magnetic intensity and mean temperature at different parts of the earth's surface; in that case, the intensity 

 seems greatest where the mean temperature is least. Perhaps, however, the St Helena Observatory is not well situated for the de- 

 termination of such a connection as a general annual law, since, in the first place, the range of temperature is small (the difference 

 between the mean temperatures of the hottest and coldest months being only 8^ Fahr., about ^ of the difference at Makerstoun) ; and, 

 in the second place, the Observatory is placed above extremely magnetic rocks ; the whole island indeed is one large magnetic mass. 

 But perhaps the result obtained from the four years' observations is the strongest objection, since it places the maximum and mini- 

 mum of magnetic intensity at the two times of yearly mean temperature. 



