INEQUALITIES OE TERRESTRIAL MAGNETISM. 
In Horizontal Force. 
423 
Tears of large 
solar curves 
Eirst Semidiurnal Wave, 0-0000596 x sine (double moon’s hour-angle +21° +15°) 
Second Semidiurnal Wave, 0-0000781 xsine (double moon’s hour-angle + 20° +15°) 
Mean, 0-0000688 x sine (double moon’s hour-angle +36°) 
Tears of small 
solar curves 
(First Semidiurnal Wave, 0-0000517 X sine (double moon’s hour-angle +34° +15°) 
-j Second Semidiurnal Wave, 0-0000588 X sine (double moon’s hour-angle +15° + 15°) 
Mean, 0-0000552 xsine (double moon’s hour-angle +41°) 
There does not appear to be any sufficient reason for concluding that one of the semi 
diurnal waves certainly differs from the other, or that the constant angle in the argument 
is certainly different in any of the several cases. But there appears to be no doubt that 
the coefficient for years of large solar curves is greater than that for years of small solar 
„ . lunar semidiurnal inequality in years of large solar curves . 
curves. e propoi ion j unar semidiurnal inequality in years of small solar curves 1S ’ 
for declination, j^y||=T35 ; 
for horizontal force, j^=T25. 
It would seem possible to suggest two conjectural reasons for this remarkable association 
in the time-law of changes of solar effect and lunar effect. One is, that the moon’s 
magnetic action is really produced by the sun’s magnetic action ; and a failure in the 
sun’s magnetic power will make itself sensible both in its direct effect on our magnets 
and in its indirect effect through the intermediation of the moon’s excited magnetism. 
The other is, that, assuming both actions (solar and lunar) to act on our magnets indi- 
rectly by exciting magnetic powers in the earth, which alone or principally are felt by 
the magnets, the earth itself may go through different stages of magnetic excitability, 
increasing or diminishing its competency to receive both the solar and the lunar action. 
The arguments of lunar inequality, in western declination from north, and in horizontal 
force to magnetic north, are sensibly the same ; so that we may consider the two dis- 
turbances to be synchronous, or that they are the effect of one disturbance in a definite 
straight line. The mean coefficient in western declination=0' , 210, which expressed in 
terms of horizontal force=(M)000611. The mean coefficient in force to magnetic north, 
similarly expressed, =0*0000621. The direction of the composite disturbing force is 
therefore in the direction magnetic N.W. and S.E. very nearly; or, in astronomical 
bearing, making an angle 65° west of the north meridian. This may be described 
roughly as in the line from the Red Sea to the south of Hudson’s Bay. 
The laws of the lunar action and the solar action are widely different. The lunar 
action is semidiurnal ; the solar action is mainly diurnal. The lunar action is in the 
N.W. direction; the solar action is mainly in the S.W. direction. (See the curve for 
the mean of years 1858-1863, in Plate XXXIV., accompanying this paper; and the 
curve for the mean of years 1848-1857, Greenwich Magnetical and Meteorological 
Observations, 1859, page clxxxv.) 
