PRESIDENTIAL ADDRESSES. 515 



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 constituents, 

 form the subjects of Schuster's paper. The con- 

 clusion 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 I hope he may be 

 induced to undertake 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 



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