( 221 ) 



Aliliotiuh lliis <',,iii|,ii(ali()ii of /\ lias only .i prcliiiiiiiary clmmctel', 

 f^t'H J ll'i"l< 1 ;i"i ïivc lo L'oiK'liule, tlial (lic ii;j,rcenic)it helwocii tlic 

 new and llie old oi)sei'\ jilioiis is as satislaclory as nuiv hc expected 

 from (he delicate nalui-(^ o!" (Ids rcsc^aicli. The |)ei-io(l we lia\e now 

 found for (he so-called 14-nion(hly motion, correspoiKlin"' wi(h a 

 yearly increase of phase of ;j()4°.l)5, even brinos out a closer agree- 

 ment between (he phases for J9()7.5 as deduced from the mo(i()n 

 before 1907 and after 1908. Widi /^.^ r= 4;UM we had found 

 a difference of 9°.4 ; reducing (he pliases found for 1905.5 and foi' 



1909.5 wiih (he above mentioned value of ' ' (o 1907 5 we obtain- 



t/t 



from 1904—1907: .f., for J907.5 — IHI^.U 



from 1908—1911: if., for 1907.5 = 184°.89. 



This resnh strengthens (he conclusion arri\ed a( on p. 216, that 

 the i.ei'(url)a(ion of 1907 did not cause any appreciable change ot phase. 



When we (ry (o lind (he cause of the decidedly rather sudden 

 change in (lu- 14-monthly nu)tion, i( seems na(ural (o seek i( in 

 the inlluence of i'a|)id displacements of mass caused for instance b\ 

 volcanic erup(ions or eai'dnpudves. }*i'of. Hki,mkrt has already developed 

 their inlluence in his "//////(/v ( ii'i)(l<isir''\ II 1\m1, S. 41(5 — 418, but 

 (he deiails of (he (tbseixcd phenomenon are no( in accoi'dauce \\i(h 

 his resulls. .\l'l(>r his analysis, a sudden dis|ilaceineu( of mass mus( 

 chielly cause a change in (he direciion of (he axis of grea(es( momen( 

 of iuei'lia, (he direc(i(m of the ins(au(ane(»us axis of rotation re- 

 maining unchanged. The angular distance between (he (wo axes is 

 allei-cd, au<l (lie axis of rota(ion condnues reg(darly i(s mo(ion in 

 a eircula!' cone around (Ih^ new pi'incipal axis of ineiiia i.e. around 

 (he u(n\' mean pole. A [dienomenon of (his characler would (here- 

 fore al(er (he posi(ion of (he mean pole and (he mean \alue of (he 

 geogra|)hical laddules, while (Ik^ analysis of (he observed fac(s did 

 on the contrary in(lica(e an unailered posidon of (he mean pole, 

 and a spii-al displacemeid of (he pole of ro(ation. 



So (he cause of the phenomenon must be souiiht elsewhere, and 

 there is anodier problem (o be sohed by (he ilynanncal (lieoi-y of 

 (he polar mo(i()n. 



Lrijdi'H Ohscrcdtorii, June 1911. 



15 

 Proceedinsrs Roval Acad. Amsterdam. Vol. XIV. 



