ON THE MAGNETIC HORIZONTAL FORCE 



7:} 



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 arc found 

 from the equation 



__ = o = + 0.40 cos (0 + 13°) + L.20 cos (2 + 39°) + 0.45 cos (3 6 + 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.) — LtTNAR-DltTRNAL VARIATION OF THE HORIZONTAL FOBCE ObSEEVED ASH COMPUTED. 



+ 1.5 



0" 1 2 3 4 5 « 7 8 9 10 11 12 13 14 15 115 17 18 19 20 21 22 23 24" 

 U. C. L. C. V. c. 



We find : Principal maximum 2 h 52 m after Upper Culmination; 4- 0.87 scale divisions. 



Secondary " 17" Lower " 4- 0.51 " " 



Principal minimum 6 41 " " " — 0.87 " " 



Secondary " S 19 " Upper " —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 1\ hours after the same phases. 



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

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

 XIX. p. 11, 1840) 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 

 hours after the lower culmination. 



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

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

 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-General Sabine 

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

 Observations, London, 1857). 



A x = + 0.05 4- 0.215 tin (<> \ 353 .6) + 0.3324 tin (2a + 13°.5). 



10 



