Vertical Component of Magnetic Force. 



245 



wintei- than in summer, as there is an indication of a minimum ahout 5^ a.m. The following are the results 

 from the mean of both years : — 



The principal minimum at 1*^ 10™ a.m., Makerstoun mean time. 



The principal maximum at 6^ p.m., 



A secondary maximum at 9'' lO'" a.m., 



A secondary minimum at ll'^ 40™ a.m., 



The same results may be deduced from these means as have been already deduced from the means 

 Table XVIII. 



Secular Change op the Vertical Component op Magnetic Force. 



The means for the year 1844 shew that the means of the observations at 22'' and 0'^, and at 4*^ and 6'\ 

 are respectively very nearly the same as the means of the observations at 23'^ and 5^. Assuming that they 

 are equal for the year 1843, we shall have from the means for the year at the foot of Table XVIII. = 0*000100 

 and from the mean for the year deduced from the means at the the foot of Table XV. = 0"0 15935. 



Mean at 20'' less than the mean of 9 observations from Table XVIII. by 0-000040, or = 0-015895 



Mean for 23*' 0-000039, or = 0-015896 



Mean at 2^ 0-000044, or = 0-015891 



Mean for 5^ greater 0-000078, or = 0-016013 



From Table XVII., Abstracts for the observations of 1842, we have the means in micrometer divisions at 

 the corresponding hours in 1842, these being converted into parts of force by the first formula given in the 

 remarks to Table XV. we have, 



1842 ; 20'^ = 0-017902, 23'^ = 0-017962, 2^ = 0-017980, 51^ = 0-018147. 



The means above for 1843 being subtracted from these, we have — 



Secular change, 201^= -0002007, 231^= -0-002066, 2^= -0-002089, 6*^= -0-002134. 

 Mean secular change 1842 to 1843, = —0-002074. 



Ranges op the Monthly Means op the Diurnal Variations. 



The ranges given in the last column of Table XVIII. must be very imperfect for some months, since the 

 minimum actually occurs in general a little after midnight. It is very obvious, however, that the range is 

 greatest at the equinoxes, and that it differs little in summer from winter, thus the diurnal range of the mean 

 for the winter group of months is 0*000204, for the summer group 0*000222, while for the group at the 

 equinoxes it is 0-000317. The corresponding groups in Table XIX. give the following ranges : — 



Diurnal range; winter group = 0-000251 ; summer group = 0000492 ; equinoctial group = 0-000788 ; 



or nearly in the ratios of 1, 2, and 3. 



TABLE XX. — Diurnal Ranges of the Vertical Component of Magnetic Force, as deduced from the 

 9 Daily Observations in 1843, with the Weekly and Monthly Means. 



vil 



January. 



February. 



March. 



April. 



May. 



June. 



July. 



August. 



September. 



October. 



November. 



December. 





0-00 



0-flO 



0-00 



0-00 



0-00 



0-00 



0-00 



o-oo 



0-UO 



0-00 



n-oo 



0-00 



1 



[ ] 



0227 



0130 



0226 



0168 



0292 



0492 



0211 





[0481] 



0442 



0400 



2 



0959 



0099 



0111 



[0865] 



0258 



0414 



[0363] 



0297 







0421 



0213 



0468 



3 



0356 



0159 



0135 



0371 



0314 



0597 



0406 



0366 



[- ] 



0501 



0681 



[0270] 



4 



0056 



0162 



0474 



0332 



0338 



0405] 



0185 



1536 





0282 



0312 



0184 



5 



0096 



[0283] 



[0810] 



3738 



0200 



0469 



0238 



0288 







1011 



[0327] 



0286 



6 



0251 



0802 



0434 



0373 



2713 



0264 



0274 



[0798] 



0262 



0396 



0217 



0166 



7 



0267 



0355 



3300 



1416 



[0894] 



0398 



0300 



0266 



0436 



0228 



0303 



0247 



8 



[0360] 



0123 



0407 



1330 



0512 



0421 



0945 



0807 



0459 



[0341] 



0232 



1177 



9 



0610 



0280 



0322 



[0838] 



1010 



0139 



[0361] 



1526 



0542 



0139 



0373 



0555 







0259 



0165 



0191 



0363 



0590 



0254 



^ 0378 



0353 



[0459] 



0133 



0246 



[0633] 



I 



0676 



0276 



0163 



0434 



0185 



[0387] 



0140 



0399 



0418 



0141 



0105 



0822 



2 



0133 



[0351] 



[0347] 



1112 



0145 



0499 



0129 



0143 



0446 



0210 



[0264] 



0552 



mag. and met. obs. 1843. 



3q 



