HoEizoNTAL Component of Magnetic Force. 



3sr 



instrument will indicate a different law in high and low latitudes.* See the discussion for the effect of 

 disturbances on the monthly means for other considerations affecting the annual variation. 



-The following are the average differences of 



Differences of the Daily Means from the Monthly Means.- 

 the daily means from their corresponding monthly means : — 



Jan. Feb. March. April. 



0000 I 191 356 548 436 



The daily means, therefore, differ least from the monthly means near the solstices, and most near the 

 equinoxes, the average difference for the latter being nearly three times that for the former. As in the case of 

 the similar discussion for the declination, this law is also related to the annual period, and may be similarly 

 genei'alized. The average difference of the daily mean fi-om the monthly mean is a minimum when the 

 horizontal component is a maximum, and vice versa. This correspondence is the more remarkable, since the 

 epochs for the greatest and least average differences are not the same for the magnetic declination and 

 horizontal component. 



The following are the averages of the positive and of the negative differences : — 



The negative departures fi-om the monthly means are greatest ; in only two months, May and J; 

 )Ositive differences of a greater value than the negative differences. 



the positive 



une. are 



TABLE XXIII. — Mean Variations of the Horizontal Component of Magnetic Force, after elimi- 

 nating the Secular Change, with reference to the Moon's Age, Declination, and Distance from 

 the Earth, for 1844. 



This Table has been formed in the same manner as Table II., the rate of secular change employed being 

 = +0-002661 per annum, or = +0*0000073 ^er diem. 



* The law for the annual period of the horizontal component has been confirmed by the discussions of the observations at Maker- 

 stoun for each of the years 1842, 1843, 1844, and 1845 ; it has also been confirmed by a discussion of the observations made at Toronto 

 in 1842 (see Transactions of the Royal Society of Edinburgh. Vol. xvi., pp. 102-103). Dr IjAMONt has not been able to conclude any 

 distinct law from the observations made at Munich (Resultate des magnetischen Observatoriuras in Miinchen, 1843 — 1845, p. 28) ; 

 this, it appears to me, is due to the mode of discussion adopted. Dr Lamont combines the monthly means into quarterly periods, 

 January, February, and March being the first, a mode of combination the best fitted to disguise the law, since each period includes 

 the whole range of the variations. Neglecting the means of the Munich Observations for 1842, which were broken up in May by 



MAG. AND MET. OBS, 1844. 4 X 



