ON THE MAGNETIC HORIZONTAL FORCE. 



73 



The curve is double-crested and is exhibited, together with the observed values, 

 in the annexed diagram. It presents two maxima and two minima, which are found 

 from the equation 



^= - = + 0.40 cos (6 + 13) + 1.20 cos (2 + 39) + 0.45 cos (30 + 245). 



The lunar effect on the declination we have found likewise to present two maxima 

 and two minima. (Sec Part III. of the discussion.) 



(B.) LUNAR-DIURNAL VARIATION OF THE HORIZONTAL FoRCE OBSERVED AND COMPUTED. 



+1.5 

 + 1.0 



+0.5 



m 



I ' 



*53 



'5 0.5 



j 



"3 



=c 1.0 

 1.5 



2.0 



23456 7 



Wo find 



0" 1 



U. C. L. C. 



Principal maximum 2 h 52 m after Upper Culmination 



9 10 11 12 13 14 15 lli 17 18 19 20 21 22 23 24" 



D. C. 



+ 0.87 scale divisions. 



Secondary " 17" Lower 

 Principal minimum 6 41 " " 

 Secondary " 8 19 " Upper 



+ 0.51 



0.87 



0.45 



The epoch of the horizontal force tide for the high values is nearly 2 hours after 

 the culminations, and for the low values it is 7| hours after the same phases. 



For Makerstoun, in Scotland, at General Sir Thomas M. Brisbane's observatory, 

 in 1843-46, Mr. J. A. Broun found (Trans. Eoyal Society of Edinburgh, Vol. 

 XIX. p. 11, 1849) the smaller maximum of the horizontal force 2 hours after 

 upper culmination, the greater maximum 1| hours after the lower culmination, the 

 smaller minimum 8 hours after the upper culmination, and the greater minimum 9 

 hours after the lower culmination. 



At Prague all extremes appear from 2 to 3 hours later. Mr. Karl Kreil (Dcnk- 

 schriften of the Imperial Academy of Sciences, at Vienna, Vol. V. 1853), found 

 from the ten year series at Prague (1840 49) maxima of horizontal force between 

 four and five hours after the upper and lower culminations, the latter being the 

 greater of the two ; and minima between ten and eleven hours after the same 

 epoch, that after the upper culmination being the greater of the two. 



From the Toronto observations, continued for five years, Major-Gencral Sabine 

 deduced the formula (see Vol. III. of the Toronto Magnetical and Meteorological 

 Observations, London, 1857). 



A,. - + 0.05 + 0.215 ein (n + 353.6) + 0.3324 sin (2 a + 13.5). 



10 



