( 208 ) 



This summary shows pretty clearly that the ampliturle was 

 found to be considerably smaller in the years 1836 — 1858 than in 

 ihe following period. For a number of series the mean errors have 

 been deduced (see A. N. 3261) ami the consideration of these 

 strengthens the conclusion which admits as probable the reality of 

 the observed difference. On the other hand no variability of the 

 amplitude is to be found after 1860 and we may conclude at least that 

 the observations make a more or less sudden change between 1850 

 and 18G0 much more probable than a periodic or continually in- 

 creasing one. Now the dynamical theory of the rotation of a sphere 

 not absolutely solid, either as a whole or in some of its parts, leads 

 to tlie same result. It teaches us,i) that with slow secular displace- 

 ments of mass the axis of the greatest moment of inertia is entirely 

 followed in its motions by the axis of rotation ; that with periodic 

 displacements the axis of rotation will get a motion of the same 

 period as that of the axis of inertia, whicli is added to its own 

 motion, but that in the case of sudden displacements of mass the 

 axis of inertia is the only one to shift its position, so that the 

 opening of the cone, described by the axis of rotation around it, 

 changes, introducing thereby a discontinuity in the motion of the latter. 

 The amplitude changes and in general also the phase, but after that 

 the motion continues in its old period. 



May we however be led in this problem by a dynamical theory? 

 Chandler denies this strenuously. He thinks it has proved itself 

 a blind guide in this case, and that he who would follow it would 

 betray reprehensible conservatism. 



It is a fact that misplaced conservatism has frequently delayed 

 the development of science and, if it were still necessary, the 

 beautiful discovery of Chandler himself of the motion of the Pole 

 named after him, would prove once again that an unprejudiced in- 

 vestigation of the observations, without being guided by any theory, 

 can lead a problem in the right paths and render an important 

 service to science. 



Bat on the other hand we are justified I think in not granting 

 the conclusion that the most simple theory is erroneous or incom- 

 plete, before such a theory is shown to be decidedly incompatible 

 with the observations. 



At the same time a theory, even a somewhat imperfect one, if it 

 be only based in general on correct foundations, is certainly entitled 



1) See a. o. Helairht. Die nuitli. luid plivs. 'J'lieorieiMi der li")lieren geodiisie. 

 Vol, II page 417. 



