Horizontal Component of Magnetic Force. 



357 



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

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



of 



Differences of the Daily Means from the Monthly Means. - 

 the daily means from their corresponding monthly means : — 



-The following are the average differences of 



Jan. 



0-000 I 191 



Feb. 



356 



March. April. 



548 436 



May. 



342 



June. 



185 



July. 



241 



Aug. 

 366 



Sept. Oct. 



278 476 



Nov. 



323 



Dec. 

 318 



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 

 generalized. The average difference of the daily mean from 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 : — 





Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



0-000 



172 



317 



445 



404 



355 



193 



217 



353 



248 



459 



280 



258 



o-ooo 



215 



404 



712 



472 



330 



178 



271 



380 



316 



494 



382 



413 



The negative departures from the monthly means are greatest ; in only two months, May and June, are 

 the positive differences of a greater value than the negative differences. 



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. 





Variation 





Variation 



After 



Variation 



After 



Variation 



Before 



Variation 



Before 



Variation 



Moon's 



of Hori- 



Moon's 



of Hori- 



Moon 



of Hori- 



Moon 



of Hori- 



and 



of Hori- 



and 



of Hori- 



Age. 



zontal 



Age. 



zontal 



farthest 



zontal 



farthest 



zontal 



after 



zontal 



after 



zontal 





Component. 





Component. 



North. 



Component. 



North. 



Component. 



Perigee. 



Component. 



Apogee. 



Component. 



Day. 



0-00 



Day. 



0(10 



Day. 



0-00 



Day. 



0-00 



Day. 



0-00 



Day. 



0-00 



15 



0133 







0607 







0499 



14 



0450 



7 



0242 



7 



0283 



16 



0172 



1 



0486 



1 



0484 



15 



0510 



6 



0164 



6 



0314 



17 



0036 



2 



0546 



2 



0589 



16 



0522 



5 



0000 



5 



0123 



18 



0120 



3 



0520 



3 



0408 



17 



0449 



4 



0131 



4 



0052 



19 



0000 



4 



0608 



4 



0259 



18 



0393 



3 



0188 



3 



0092 



20 



0203 



5 



0572 



5 



0057 



19 



0339 



2 



0206 



2 



0065 



21 



0285 



6 



0489 



6 



0148 



20 



0333 



1 



0202 



1 



0079 



22 



0320 



7 



0506 



7 



0175 



21 



0151 



P 



0341 



A 



0164 



23 



0301 



8 



0510 



8 



0255 



22 



0200 



1 



0366 



1 



0280 



24 



0452 



9 



0235 



9 



0259 



23 



0210 



2 



0305 



2 



0455 



25 



0526 



10 



0067 



10 



0245 



24 



0000 



3 



0271 



3 



0473 



26 



0492 



11 



0130 



11 



0436 



25 



0199 



4 



0383 



4 



0457 



27 



0606 



12 



0008 



12 



0387 



26 



0129 



5 



0365 



5 



0433 



28 



0550 



13 



0119 



13 



0471 



27 



0339 



6 



.0439 



6 



0218 



29 



0600 



14 



0028 











7 



0203 



7 



0070 



This Table has been formed in the same manner as Table II. 

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



the rate of secular change employed being 



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

 Btoun 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 Lamont has not been able to conclude any 

 distinct law from the observations made at Munich (Resultate des magnetischen Observatoriums 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 



