Diurnal Variations for the Vertical Component of Magnetic Force. Ivii 



the mean positive disturbance occurred about 6^ p.m., the minimum between 11'^ p.m. and 8^ a.m. ; that the 

 maximum of the mean negative disturbance occurred about 2^ a.ji. and the minimum about 6'' p.m., although 

 the values varied little between noon and 8 p.m. 



109, The approximate epochs of maxima and minima for the mean disturbance, independent of sign, (indi- 

 cating the principal maximum by + and minimum by — ), are as follow : — 



Min. 



Max. 



Min. 



Max 



llh A.M. 



+ 6'^ P.M. 



9l> P.M. 



Ih A.M. 



IQl' A.M. 



5^^ P.M. 



8^^ P.M. 



+ 1^^ A.M. 



I'' P.M. 



5" P.M. 



-101> P.M. 



+ 3*^ A.M 



llh A.M. 



5'^ P.M. 



811 p M_ 



+ Sf" A.M 



Winter — Nov. Dec. Jan. 

 Spring — Feb. March, April, 

 Summer — May, June, July, 

 Autumn — Aug. Sept. Oct. 



It has been seen that the positive disturbance has its maximum about 5*^ — 6^ p.m., and the negative dis- 

 turbance its maximum about 2^ a.m., so it appears now that the mean disturbance, independent of sign, has a 

 maximum near both hours. 



110. In winter the secondary maximum, about l'^ a.m., is very indistinctly marked ; in summer the second- 

 ary maximum, about 5^ p.m., is very small compared with the other ; the principal minimum occurs in winter 

 about ll** A.M. ; that at 10"^ p.m. is best marked in summer. In this case also, as in the cases of the magnetic 

 declination, No. 45, and the horizontal component, No. 77, the diurnal variation of the magnetic disturbance 

 in summer is nearly the reverse of that in luinter. The diurnal variation of the disturbance is very nearly the 

 same in spring as in autumn. 



Table 54. — Variations of the Vertical Component of Magnetic Force with reference to the Moon's 

 Hour-Angle for the Winter and Summer Lunations, and for all the Lunations of the Years 1844 

 and 1845. 



Moon's 

 Hour-* 

 Angle. 



Winter Lunations. 



Summer Lunations. 



All the Lunations. 



1844. 



1845. 



Mean. 



1844. 



1845. 



Mean. 



1844. 



1845. 



Mean. 



h. m. 



0-00 



0-()0 



000 



0-00 



0-00 



0-00 



0-00 



0-00 



0-00 







-0054 



+ 0002 



-0026 



+ 0017 



+ 0007 



+ 0012 



-0019 



+ 0005 



-0007 



2 25 



-0086 



+ 0013 



-0036 



-0009 



+ 0015 



+ 0003 



-0047 



+ 0014 



-0016 



4 20 



-0069 



-0039 



-0054 



-0014 



+ 0013 



0000 



-0042 



-0015 



-0028 



6 15 



-0071 



-0029 



-0050 



-0022 



+ 0002 



-0010 



-0046 



-0014 



-0030 



8 10 



-0058 



-0031 



-0044 



-0031 



-0019 



-0025 



-0045 



-0025 



-0035 



10 5 



1 -0007 



+ 0002 



-0002 



-0009 



-0001 



-0005 



-0008 



+ 0001 



-0003 



12 



'' +0059 



+ 0030 



+ 0045 



-0018 



+ 0010 



-0004 



+ 0020 



+ 0021 



+ 0020 



13 55 



+ 0107 



+ 0038 



+ 0072 



+ 0007 



+ 0020 



+ 0013 



^ + 0058 



+ 0030 



+ 0044 



15 50 



+ 0094 



+ 0036 



+ 0065 



+ 0039 



+ 0001 



+ 0020 



+ 0067 



+ 0020 



+ 0043 



17 45 



+ 0049 



+ 0021 



+ 0035 



+ 0017 



-0020 



-0001 



+ 0033 



+ 0002 



+ 0017 



19 40 



+ 0043 



-0014 



+ 0014 



+ 0007 



-0028 



-0010 



+ 0025 



-0021 



+ 0002 



21 35 



1 -0004 



i 



-0025 



-0014 



+ 0015 



0000 



+ 0007 



+ 0006 



-0013 



-0003 



111. Variation of the Vertical Component with reference to the Moon's Hour-Angle. — There are four 

 independent results in Table 54, namely two for the vnnter lunations, and two for the summer lunations of 

 1844 and 1845, the others depend on these. In all the four the maximum vertical component occurs between 

 2 and 4 hours after the moon's transit of the inferior meridian ; in three cases a secondary maximum occurs at, 

 or shortly after the superior transit, — minima occurring during the intermediate period, from 6 to 8 hours after, 

 and from 2 to 4 hours before, the superior transit; in the winter lunations for 1844, only the principal maxi- 

 mum and minimum are shewn (see the similar case for the horizontal component No. 80), and, as the variations 

 for this group are much greater than for any of the others, it is probable that the difference is due to disturb- 

 ances. See the volumes for 1844, p. 382, where the elimination of the larger disturbances leaves traces of a 

 secondary maximum and minimum. 



112. From the means of all the winter lunations in Table 54 



The vertical component is a maximum about 2 hours after the moon's inferior transit. 

 minimum 5 hours after the moon's superior transit. 



