Solar-diurnal Variation of the Magnetic Declination at Pekin. 479 



of the annual mean, by reason of a particular feature of the semi- 

 annual inequality spoken of above. When, on the other hand, we 

 direct our attention to the semiannual mean from October to March, 

 we see the consequence of the superposition upon the annual mean 

 of the opposite semiannual inequality belonging to these months : 

 this is most particularly shown in the effect produced upon the semi- 

 annual mean by the great semiannual loop which culminates about 

 6 or 7 a.m. This deflection, which is opposite in direction to that 

 appropriate to the hemisphere, prevails over it so far as to interrupt 

 the progression, which on the mean of the year is continuous, and to 

 produce a secondary maximum at about 7 a.m. This opposite de- 

 flection to that which is normal in the hemisphere, taking place at 

 the hours when the semiannual inequality is greatest, is a common 

 feature whenever, in the middle latitudes of either hemisphere, the 

 mean diurnal variation of the one hemisphere is combined with the 

 semiannual inequality which has the opposite analogy. The hour 

 of principal discordance between them is always nearly the same, 

 being determined by that of the principal deflection on the semiannual 

 curve, which, as seen in the diagram, is nearly identical in solar time 

 in all parts of the globe. In the semiannual mean from October to 

 March the principal turning hours are a little later than in the an- 

 nual mean, just as we have seen above that, in the April to September 

 mean, the turning hours are a little earlier than in the annual, and 

 for the same reason. Finally, it is this combination of the hemi- 

 spherical and semiannual effects which creates the differences we ob- 

 serve in the amount and hours of the solar- diurnal variation in the 

 different months in the middle latitudes of both hemispheres. 



There is one more feature of some importance to the general 

 theory of the diurnal variation, which is illustrated by the Pekin obser- 

 vations, and requires a brief notice. A distinction has been elsewhere 

 pointed out (Cosmos, English Translation, Longman's Edition, vol. 

 iv. p. 504, Editor's Note) between the diurnal variation of the equa- 

 torial zone and that of the middle latitudes, consisting in the circum- 

 stance that in the equatorial zone the amount of the semiannual de- 

 flection is greater than that of the hemispherical deflection at the 

 hours when they are opposed to each other, and by its preponderance 

 changes the character, instead of simply diminishing the amount, 

 of the hemispherical deflection. The change in the signs of the de- 

 flection at 6 and 7 a.m. in the semiannual mean from October to 

 March at Pekin is an illustration of this peculiarity, and ought 

 perhaps in strictness to cause Pekin to be included in the magne- 

 tically equatorial zone ; but being only just within the border, it has 

 been found more convenient to dwell on this occasion upon the fea- 

 tures which it has in common with stations in the middle latitudes. 



The diurnal variation at Pekin reaches its extreme deflections at 

 the same hours of solar time, as is the case at the other stations in 

 the northern hemisphere where the phenomena have been examined 

 with equal care. This fact is not in accord with the opinions of those 

 physicists who regard the solar action as conditioned in its exercise 

 by the direction of the magnetic meridian at the particular station. 



