1892.] President's Address. 305 



inversely as the cube of the earth's distance from the sun. The com- 

 ponents of this force parallel and perpendicular to the earth's axis 

 are, respectively, 0'92 and O4 of the whole ; of which the former could 

 only be perceived in virtue of the varying distance of the earth from 

 the sun in the course of a year ; while the latter would give rise to 

 n daily variation, the same as would be observed if the red ends of 

 terrestrial magnetic needles were attracted towards an ideal star of 

 declination and right ascension 270. Hence, to discover the disturb- 

 ances of terrestrial magnetism, if any there are, which are due to 

 direct action of the sun as a magnet, the photographic curves- of the 

 three magnetic elements given by each observatory should be 

 analysed for the simple harmonic constituent of annual period and 

 the simple harmonic constituent of period equal to the sidereal day. 

 We thus have two very simple problems, each of which may be 

 treated with great ease separately by a much simplified application of 

 the principles on which Schuster has treated his much more complex 

 subject, according to Gauss' theory as to the external or internal 

 origin of the disturbance, and Professor Horace Lamb's investigation 

 of electric currents induced in the interior of a globe by a varying 

 external magnet. The sidereal diurnal constituent which forms the 

 subject of the second of these simplified problems is smaller, but not 

 much smaller, than the solar diurnal term which, with the solar 

 semi-diurnal, the solar ter-diurnal, and solar quarter-diurnal con- 

 stituents, form the subjects of Schuster's paper. The conclusion at 

 which he has arrived, that the source of the disturbance is external, 

 is surely an ample reward for the great labour he has bestowed on 

 the investigation hitherto ; and 1 hope he may be induced to under- 

 take the comparatively slight extension of his work which will be 

 required for the separate treatment of the two problems of the 

 sidereal diurnal and the solar annual constituents, and to answer for 

 each the question : Is the source external or internal ? 



But even though external be the answer found in each case, we must 

 not from this alone assume that the cause is direct action of the sun 

 as a magnet. The largeness of the solar semi-diurnal, ter-diurnal, 

 and quarter-diurnal constituents found by the harmonic analysis, 

 none of which could be explained by the direct action of the sun as a 

 magnet, demonstrate relatively large action of some other external 

 influence, possibly the electric currents in our atmosphere, which 

 Schuster suggested as a probable cause. The cause, whatever it may 

 be, for the semidiurnal and higher constituents would also probably 

 have a variation in the solar diurnal period on account of the 

 difference of temperature of night and day, and a sidereal and animal 

 period on account of the difference of temperature between winter 

 and summer. 



Even if, what does not seem very probable, we are to be led by the 



