DIURNAL VARIATIONS FOR THE VERTICAL COMPONENT OF MAGNETIC FoRCE. lh 
100. When we combine the means in groups in the manner already adopted for the magnetic declination, 
No. 31, we obtain the following epochs of maxima and minima in apparent time (see Plate III.) 
Dec. Jan. Feb. March, April, May, June, July, Aug. Sept. Oct. Nov. 
Maximum, 55 50™ pm. 54 40™ p.m. 6> Om p.m. 55 40™ p.m. 45 25™ p.m. 
Minimum, 3h 40™ aM. 12> 10™ a.m. 1h 45m a.m. 2h 10™ a.m. 25 30™ a.m. 
Maximum, 8» 40™ a.m. 8 Om a.m. 8h 35™ a.m. 
Minimum, 02 0™ Noon. 0h O™ Noon. 115 45™ a.m, 
The conclusions from the epochs for these groups are quite similar to those already obtained from the epochs 
for the single months. The afternoon maximum occurs earliest in September to November. 
TABLE 50.—Diurnal Variations of the Vertical Component of Magnetic Force for Different Periods, 
deduced from Days selected as free from Irregular Disturbances, in the Years 1844 and 1845. 
| | Six Months. | 
Mak. Dec. March. May. July. Sept. —________——_|| Twelve 
cae | phe April. June. Aug. ae Sept. to March Months, 
Feb. to Aug. 
h m 0-00 000 0) | 0-00 0-00 0-00 0-00 
12 13 —0018 | —0065 | —0063 | —0069 | —0079 || —0048 | —0066 — 0057 
13 13 || —0023 | —0069 | —0056 | —0071 | —0081 —0052 | —0065 — 0058 
14 13 || —0028 | —0038 | —0038 | —0060 | —0070 || —0049 | —0045 — 0047 
15 13 | —0035 | —0026 | —0003 | —0031 | —0067 —0051 | —0020 || —0035 
16 13 || —0040 | —0032 | +0030 | +0011 | —0063 —0051 | +0003 — 0024 
17 13 —0045 |, —0022 | +0060 | +0038 | —0057 —0051 | +0025 — 0013 
18 13 — 0049 | +0003 | +0077 | +0068 | —0043 —0046 | +0049 || +0001 
19 13 —0050 | +0029 | +0083 | +0075 | —0013 —0031 | +0062 || +0016 
20 13 —0035 | +0058 | +0071 | +0058 | +0014 || —0010 | +0062 || +0026 
21 13 | —0034 | +0052 | +0017 | +0030 | +0021 || —0006 | +0033 || +0013 
—0014 | —0034 | —0112 | —0071 | —0012 | —0013 | —0072 || —0043 
—0003 | —0084 | —0126 | —0099 | —0018 | —0010 | —0103 || —0057 
+0015 | —0058 | —0099 | —0081 | +0014 || +0014 | —0079 || —0032 
— 0034 || +0010 
|| +0065 | +0026 | —0007 | +0018 | +0106 || +0085 | +0012 ||- +0049 
13 || +0066 | +0070 | +0045 | +0071 | +0119 || +0092 } +0062 || +0077 
13 || +0049 |} +0083 | +0072 | +0086 | +0100 | +0074 | +0080 || +0077 
| +0075 || +0065 
| +0039 | +0051 | +0065 | +0049 | +0049 || +0044 | +0055 || +0049 
+0036 | +0033 | +0051 | +0031 | +0038 || +0037 | +0038 || +0037 
13 | +0038 | +0020 | +0018 | —0005 | +0004 || +0021 | +0011 || +0016 
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101. When we consider the diurnal variation as deduced from days selected as nearly free from inter- 
mittent disturbance (No. 85), and as exhibited in the means, Table 50, and the dotted curves, Plate ITI., we 
find that the epochs of maximum are considerably altered as well as the whole form of the diurnal curve. 
The epochs of maxima and minima in apparent time are as follow :— 
Dec. Jan. Feb. March, April, May, June, July, Aug. Sept. Oct. Nov. 
Maximum, 35 40™ p.m. 5 10™ p.m. 6" 0™ p.m. 5> 10™ p.m. 4h 10™ pm. 
Minimum, 65 40™ a.m. 12 0m a.m. 122 20™ a.m. 125 40™ a.m. 125 55™ am. 
Maximum, 8» 30™ a.m. 7h Om a.m. 6" 55™ aM, 9» 10™ a.m. 
Minimum, 0h 20™ p.m. 0 Om Noon. © 0? 20™ p.m. 02 5™ pm. 
Hence, in the undisturbed diurnal variation, the maximum of the vertical component occurs earliest in the 
months December to February, and latest in the months May and June. 
The form of the diurnal curve is quite different in the months November to February, from that for the 
other months : in the four winter months the diurnal curve is single, having but one maximum and minimum, 
