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. Feb. March. April. May. 



0-000 I 322 121 117 309 212 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



218 



090 



000 



053 



048 



233 



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. 





Variations 





Variations 



After 



Variations 



After 



Variations 



Before 



Variations 



Before 



Variations 



Moon's 



of 



Moon's 



of 



Moon 



of 



Moon 



of 



and 



of 



and 



of 



Age. 



Total 



Age. 



Total 



farthest 



Total 



farthest 



Total 



after 



Total 



after 



Total 





Force. 





Force. 



North. 



Force. 



North. 



Force. 



Perigee. 



Force. 



Apogee. 



Force. 



Day. 



0-00 



Day. 



0-00 



Day. 



0-00 



Day. 



0-00 



Day. 



0-00 



Day. 



o-oo 



15 



0000 







0167 







0149 



14 



0115 



7 



0133 



7 



0087 



16 



0038 



1 



0113 



1 



0112 



15 



0080 



6 



0100 



6 



0123 



17 



0063 



2 



0125 



2 



0120 



16 



0090 



5 



0000 



5 



0066 



18 



0068 



3 



0115 



3 



0106 



17 



0126 



4 



0072 



4 



0049 



19 



0020 



4 



0120 



4 



0053 



18 



0095 



3 



0075 



3 



0055 



20 



0120 



5 



0146 



5 



0002 



19 



0055 



2 



0074 



2 



0090 



21 



0117 



6 



0128 



6 



0043 



20 



0097 



1 



0067 



1 



0060 



22 



0111 



7 



0149 



7 



0071 



21 



0031 



P 



0075 



A 



0080 



23 



0148 



8 



0137 



8 



0067 



22 



0033 



1 



0084 



1 



0131 



24 



0165 



9 



0117 



9 



0077 



23 



0097 



2 



0101 



2 



0166 



25 



0153 



10 



0020 



10 



0033 



24 



0000 



3 



0079 



3 



0125 



26 



0140 



11 



0066 



11 



0065 



25 



0073 



4 



0096 



4 



0130 



27 



0142 



12 



0016 



12 



0042 



26 



0060 



5 



0178 



5 



0136 



28 



0084 



13 



0033 



13 



0114 



27 



0109 



6 



0127 



6 



0120 



29 



0115 



14 



0012 











7 



0091 



7 



0084 



* 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 Makerstoun 

 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 J 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. 



