122 ANNUA... OF SCIENTIFIC DISCOVERY. 



suj . n of these two systems of diurnal fluctuation gives rise to a 



series of compound variations analogous to the superposition of two undula- 

 tions having the same period, but different amplitudes, and different epochal 

 times. And that, by attending to this principle, many of the most complex 

 phenomena, such as that of a double maximum and minimum, with the 

 occurrence of a nightly as well as a daily movement, are explained in a sat- 

 isfactory manner. 



The discussion of the observations already accumulated has further 

 brought into view, and, in the opinion of your committee fully established, 

 the existence of a very extraordinary periodicity in the extent of fluctuation 

 of all the magnetic elements, and in the amplitude and frequency of their 

 irregular movements especially, which connects them directly with the phys- 

 ical constitution of the sun, and with the periodical greater or less prevalence 

 of spots on its surface the maxima of the amount of fluctuation corres- 

 ponding to the maxima of the spots, and these again with those of the exhi- 

 bitions of the Aurora Borealis, which appears also to be subject to the same 

 law of periodicity ; a law which, as it does not agree with any of the other- 

 wise known solar, lunar, or planetary periods, may be considered as, so to 

 speak, personal to the sun itself. And thus we find ourselves landed in a 

 system of cosmical relations, in which both the sun and the earth, and prob- 

 ably the whole planetary system, are implicated. 



That the sun acts in influencing the earth's magnetism, in some other 

 manner than by its heat, seems to be rendered very probable by several fea- 

 tures of this inquiry; and the idea of a direct magnetic influence exterior to 

 the earth, is corroborated by the discovery of a minute fluctuation in the 

 magnetic elements, having for its period not the solar but the lunar day, 

 and, therefore, directly traceable to the action of the moon. The detection 

 of this fluctuation by Mr. Kreil, from a discussion of the Prague observa- 

 tions, has been confirmed by the evidence afforded by those of our colonial 

 observatories, and appears to be placed beyond all question by the recent 

 deductions from the horizontal force and the declination extending over 

 three years of observation at the Cape of Good Hope, which General Sabine 

 has submitted for your committee's inspection, and in both which the fluc- 

 tuations in question emerge in a very satisfactory manner, and one calcu- 

 lated to give a high idea of the precision of which such determinations are 

 susceptible, when it is considered that the total amplitude of oscillation due 

 to this cause in the direction of the Cape needle is only about 16" of angle. 



The committee also quote the following extract from a communication 

 addressed to them by Gen. Sabine, on the importance of continuing the sys- 

 tem of observations at the present time : 



" Recent observations in North America, discussed in the proceedings of 

 the Royal Society for January the 7th, 1858, have made known that the gen- 

 eral movement of translation of the isoclinal and isogonic lines, which from 

 the earliest observations have been progressing from west to east, has within 

 a few years reached its extreme eastern oscillation, and that the movement 

 in the reverse direction has already commenced; we live, therefore, at an 

 epoch in the history of terrestrial magnetism, which, we have reason to 

 believe, will be regarded hereafter when theory shall have more advanced 

 as a highly important and critical epoch. The geographical position of 

 the maximum force in the northern hemisphere appears to have reached its 

 extreme easterly elongation, and from this time forth may be expected to 

 move for many years to come towards the meridian which it occupied in 



