(51 ) 



„Report etc." pag-e 11 — 12). Yet I believe, especially on account 

 of the good mutual agreement of the results 1890 —92 and 1893 — • 

 9G, obtained bv the cooperation of observatories partly different for the 

 two periods, that the results obtained are on the whole trustworthy. 

 To derive the yearly term H G. v. d. S. Bakiiuyziïx also exi^lu- 

 sivelv made use of the results then known of the Horrebow-obser- 

 vations, namely the series of observations of Berlin, Potsdam, Prague 

 and Strasburg made in 1889 — 1892. Of course only the ^ co-ordinate 

 can be derived from these and, computing the corrections for the 

 small differences in longitude by considering the motion as circular, 

 the result is : 



«-Sept. 12 

 -j-0".112 fos2 ;i 



365 



nearlv coinciding with that which has now been derived from so 

 much fuller data. 



Let us finally consider what may be said about the meaning of 

 the results obtained. In 1890 Raüau i) for the first time drew atten- 

 tion to the fact that by cooperation of periodic displacements of mass, 

 depending on the season, with the period proper of the axis of the 

 earth, then evaluated at about 304 days, motions of the axis of 

 inertia, caused by the above mentioned displacements, may be trans- 

 mitted greath' magnified to the axis of rotation. By the relatively 

 small difference between the period of the perturbing action and the 

 period proper, we have to do with phenomena which can be regard- 

 ed as resonance and consequently the possibility arises that the 

 relatively small displacements of mass which may be brought about 

 b\ meteorological influences, may cause a rather considerable motion 

 of the axis of rotation. Shortly afterwards the matter was more 

 fully investigated by Helmert -) who showed that the general el- 

 li[)tic motion of the pole of inertia causes motions of the pole of 

 rotation in orbits, which in the various cases may have every shape 

 from the circle with direct motion through more and more excentric 

 ellipses and the straight line, to ellipses with retrograde motion and 

 finally the retrograde circle. In the most favourable case (a direct 

 circular motion of the pole of inertia) the motion is transmitted mag- 

 nified about 6 times, in the most unfavourable case (a retrograde 



1) Bulletin astron. T. Ylt, page 352. 



2) Astrou. Nachr. Vol. 120, X". 3014. 



Proceedings Roya' Acad. Amsterdam. Vol. I. 



