DiuENAL Variations for the Vertical Component of Magnetic Force. 1 



111 



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. 



Maximum, 

 Minimum, 

 Maximum, 

 Minimum, 



.f^h 



50^ 



3h 40m A.M. 



March, April, 



5I1 40m P.M. 



12^ 10™ A.M. 



&" 40m A.M. 



Qh o-" Noon. 



May, June, 

 6^ Om P.M. 

 Ih 45m A.M. 

 &" 0°i A.M. 

 Oh Qm Noon. 



July, Aug. 



5h 40m P.M. 



2^ 10™ A.M. 



&" 351° A.M. 



llh 45™ A.M. 



Sept. Oct. Nov. 

 4h 25m P.M. 

 2^ SO"" 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. 

 Mean 

 Time. 



Dec. 

 Jan. 

 Feb. 



March. 

 April. 



May. 

 June. 



July. 

 Aug. 



Sept. 

 Oct. 

 Nov. 







Twelve 

 Months. 



Sept. to 



March 

















Feb. 



to Aug. 





h. 



m. 



0-00 



O'OO 



O'OU 



0-00 



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 



22 



13 



-0025 



+ 0017 



-0047 



-0004 



+ 0002 



-0011 



-0011 



-0011 



23 



13 



-0014 



-0034 



-0112 



-0071 



-0012 



-0013 



-0072 



-0043 







13 



-0003 



-0084 



-0126 



-0099 



-0018 



-0010 



-0103 



-0057 



1 



13 



+ 0015 



-0058 



-0099 



-0081 



+ 0014 



+ 0014 



-0079 



-0032 



2 



13 



+ 0043 



-0008 



-0053 



-0040 



+ 0068 



+ 0055 



-0034 



+ 0010 



3 



13 



+ 0065 



+ 0026 



-0007 



+ 0018 



+ 0106 



+ 0085 



+ 0012 



+ 0049 



4 



13 



+ 0066 



+ 0070 



+ 0045 



+ 0071 



+ 0119 



+ 0092 



+ 0062 



+ 0077 



5 



13 



+ 0049 



+ 0083 



+ 0072 



+ 0086 



+ 0100 



+ 0074 



+ 0080 



+ 0077 



6 



13 



+ 0040 



+ 0070 



+ 0078 



+ 0077 



+ 0072 



+ 0056 



+ 0075 



+ 0065 



1 



13 



+ 0039 



+ 0051 



+ 0065 



+ 0049 



+ 0049 



+ 0044 



+ 0055 



+ 0049 



8 



13 



+ 0036 



+ 0033 



+ 0051 



+ 0031 



+ 0038 



+ 0037 



+ 0038 



+ 0037 



9 



13 



+ 0038 



+ 0020 



+ 0018 



-0005 



+ 0004 



+ 0021 



+ 0011 



+ 0016 



10 



13 



+ 0019 



-0021 



-0022 



-0029 



-0026 



-0003 



-0024 



-0014 



11 



13 



-0014 



-0053 



-0052 



-0063 



-0072 



-0043 



-0056 



-0049 



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 III., 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 : — 



Maximum, 

 Minimum, 

 Maximum, 

 Minimum, 



Dec. Jan. Feb. 

 3*" 40™ P.M. 

 &" 40™ A.M. 



March, April, 

 5h iQin P.M. 

 V" 0™ A.M. 

 SJ' 30™ A.M. 



Ot 20™ P.M. 



May, June, 



&" 0™ P.M. 



12h 20™ A.M. 



T** 0™ A.M. 



Oh 0™ Noon. 



July, Aug. 



5*^ 10™ P.M. 

 12*^ 40™ A.M. 



&" 55™ A.M. 



O^^ 20™ P.M. 



Sept. Oct. Nov. 



4"^ 10™ P.M. 



12'» 55™ A.M. 



9" lO™ A.M. 



O'' 5™ P.M. 



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, 



