HORIZONTAL FORCE OE THE EARTH’S MAGNETISM. 
399 
Table IV. — Cases in which the Earth’s Magnetic Force diminished one thousandth of 
its whole value or more in 1844 and 1845. 
No. of 
case. 
Dates of beginning and ending. 
Change of 
force in 
hundred 
thousandths. 
Solar meridians. 
5 to 10. 
11 to 14. 
-3 to +1 
and others. 
i. 
1844. Mar. 28 to Mar. 30 
-360 
+ 8 to +10 
o 
104 
+ 11 to +12 
3. 
May 21 „ May 23 
107 
+ 11 „ +12 
4. 
July 8 „ July 9 
116 
+ 7 to + 8 
5. 
Aug. 1 „ Aug. 2 
175 
+ 5 „ + 6 
6. 
Ang. 9 .. Aim. 1 0 . 
135 
+ 13 „ +14 
Sept. 24 „ Sept. 27 
115 
+ 7 to +10 
8. 
Sept. 30 .. Oct. 1 
104 
+ 13 „ +14 
9. 
Oct. 19 „ Oct. 21 
. 268 
+ 6 to + 8 
10. 
Nov. 19 .. Nov. 21 
130 
+ 13 „ +14 
11. 
Dec. 27 „ Dec. 30 .... 
165 
-3 to 0 
12. 
1845. Jan. 8 „ Jan. 10 
210 
+ 9 to +11 
13. 
Jan. 18 ,, Jan. 20 ... 
163 
(-6 „ -4) 
14. 
Eeb. 20 „ Feb. 21 
118 
0 „ +1 
15. 
Apr. 12,. Apr. 14 
350 
— 1 „ +1 
16. 
June 10 „ June 12 
110 
6 to -f- 8 
17. 
July 24 „ July 25 
154 
—2 „ —1 
18. 
July 31 „ Aug. 3 
102 
+ 6 to + 9 
19. 
Aug. 26 „ Aug. 27 
101 
+ 5 „ + 6 
20. 
Aug. 29 „ Aug. 30 
159 
+ 8 „ + 9 
21, 
Sept. 24 „ Sept. 25 
153 
+ 8 „ + 9 
22. 
Oct. 8 „ Oct. 10 
126 
(-4 „ —2) 
23. 
Oct. 20 „ Oct. 21 
118 
+ 8 to + 9 
24. 
Oct. 30 „ Nov. 1 
100 
(-8 „ -6) 
25. 
Nov. 15 „ Nov. 18 
100 
+ 8 to +10 
26. 
Nov. 27 » Nov. 29 
110 
( — 6 „ -4) 
27. 
Dec. 3 „ Dec. 4 ...... 
291 
0 „ +1 
28. 
Dec. 11 „ Dec. 13 
122 
+ 8 to +10 
An examination of this Table will show that nearly half of the great changes began 
when the 8th meridian after the zero had passed, while five began near the 12th after, 
and five near the zero itself. The two greatest changes of magnetic force in 1844 
happened near the + 8 meridian, and the two greatest in 1845 at the 0 meridian*. 
* Kkeil, who obtained so many interesting results, has noticed the fact that great disturbances occurred 
frequently on the same, or nearly the same, day of successive years. “ Thus the greatest disturbances which 
took place in the year 1836 were on the 22nd and 23rd April and on the 18th October, and both these were 
repeated on the same days in 1837.” In 1838 several disturbances occurred within a few days of the same dates 
of 1837, a list of which he gives. He also draws attention to the fact of “ the symmetrical arrangement of these 
disturbances in the same year, many of them being nearly six months apart” (Phil. Mag. 1840, vol. xvi. p. 246). 
It has been pointed out that 14 solar rotations are performed in 364 days, or within one day of the year ; there 
are also 7 rotations in six months. The recurrence exactly after twelve or six months does not appear for the 
years 1844 and 1845, hut of the five greatest changes of daily mean horizontal force (Cases Nos. 1, 9, 12, 15, 
and 27) the first two were at an interval of 8 periods of 26 days, the second and third at an interval of 3 periods 
of 26 days, and the fourth and fifth at an interval of 9 solar rotations. As, however, the disturbances obey 
also an annual law with the maxima near the equinoxes (as I first pointed out in 1847), the probability is the 
greatest of disturbances recurring at the end of 7 rotations (six months), or 12 rotations (one year), near the 
equinoxes. 
