———— 
DIuRNAL VARIATIONS FOR THE MAGNETIC DECLINATION. XXVIl 
TABLE 18.—Mean Difference of the Observations of Magnetic Declination in 1844 and 1845 from 
the Monthly Means, at the corresponding Hour in each Year, as deduced from all the Regular 
Observations. 
Mean Westerly Difference. 
Mean Hasterly Difference. Mean Difference. 
Nov. | Feb. | May. | Aug. Noy. . | May. | Aug. - | Feb. | May. | Aug. | 
| Mar. | June. | Sept. ar. | Dec. y. | June. | Sept. || Year. . | Mar. | June. | Sept. 
.| July. | Oct. Jan. il.| July. | Oct. an. | April.| July. | Oct. 
| / = 
| 
’ , 
1-47| 1-80 || 1-62 -54| 1-80| 2-71 | -83| 2-36| 1-62] 2-16] 
1-71|| 1- 16) 3- 2:07] 2:51] 2- . . : 2-03 | 
2-01 -66 8 : 1-77 | 2:34 || 2.28 : E : 2-16 
2.04 6 : + 1-36 | 1-94 
1-63 || 1- : é 1-36] 1-63 
2.73 4 + . 1-41} 1-49 
3-84 |) 1- . . 1-25] 1-39 
3-00 || 1- : : 1-21} 1-47 
2.90 F : : 1-28} 1-64 
2-67 || 2. . -24| 1-37] 1-59 
1-93 || 1. . -06| 1-31] 1-50 
1-64 || 1- -23| 1-10] 1-51] 1-38 
: 1-29] 1-33 
1-39] 1-56 
1-52 
Dee eee eee 
SAAROK ODN WwW N 
NONwWWOOWDNre YU 
SOU RR eK 
Oe DUNO 
SR OODOONANKRWNHOH SO 
— 
43. Diurnal Variation of the Mean Excursions of the Declination Magnet, from the Monthly Mean Positions 
for cach Hour, from the Observations for 1844 and 1845.—In the previous investigations, we have considered 
the effect of irregular disturbance on the hourly mean position, the frequency of positive and negative exeur- 
sions, and the sums of the latter referred to the hourly means of selected days ; we have still to consider the 
mean values of the excursions which may evidently follow different laws from the sums, as the latter may de- 
pend upon both the number and mean value. Table 18 has been formed in the following manner: Half the 
sums of the differences of the hourly observations from the monthly means for the corresponding hours being 
positive and half negative, half the sums were divided by the number of positive excursions to obtain the first 
portion of Table 18, and by the number of negative excursions to obtain the second portion; the third portion 
is obtained by dividing the whole sums by the whole number of observations. Table 19 has been formed simi- 
larly, excepting that the sums of the positive and negative disturbances are unequal (See Makerstoun Observa- 
tions for 1844, p. 350). The quantities in Table 18 have been termed mean differences, those in Table 19, 
mean disturbances ; the former being related to the means for all the observations, the latter to the means of 
the undisturbed days. The epochs of maximum and minimum are nearly the same for both Tables ; those from 
Table 19 only are given, as it is the best exponent of the laws with reference to approximate normal mean 
positions. 
Mean Westerly Disturbance. Mean Easterly Disturbance. Mean Disturbance. 
Max. Min. Max. Min. Max. Min. 
Noy., Dec., Jan., 1° p.m. 10° p.m. 6h_9} p.m. 8" aM. 62-11" p.m. 5 aM. 
Feb., Mar., April, 2h p.m. 65114 p.m. 9b p.m. 7>_10 am. 91 p.w.—1) a.m. 6" aM. 
May, June, July, 84—11> a.m. 95 p.m. 1235 4m. 8am. & 55 p.m, 1232 a.m. & 11" a.m. 5-10" pm. 
Aug., Sept., Oct., 83> a.m. 92 pm. 10° pm. 82 am. 10° P.M. 5) pm, 
44, The epochs given above can be considered only roughly approximative, since the value of the average 
excursion for the hours about the times noted often varies very slowly. The points of most consequence in 
these results are as follow :— 
MAG. AND MET. oBs. 1845 anp 1846. g 
