ON A DYNAMICAL TOP. 261 



This result, however, is to be regarded as very doubtful, as there did not 

 appear to be evidence for any variation exceeding half a second of space, and 

 more observations would be required to establish the existence of so small a 

 variation at alL 



I therefore conclude that the earth has been for a long time revolving 

 about an axis very near to the axis of figure, if not coinciding with it. The 

 cause of this near coincidence is either the original softness of the earth, or 

 the present fluidity of its interior. The axes of the earth are so nearly equal, 

 that a considerable elevation of a tract of country might produce a deviation 

 of the principal axis within the limits of observation, and the only cause which 

 would restore the uniform motion, would be the action of a fluid which would 

 gradually diminish the oscillations of latitude. The permanence of latitude essen- 

 tially depends on the inequality of the earth's axes, for if they had been all 

 equal, any alteration of the crust of the earth would have produced new prin- 

 cipal axes, and the axis of rotation would travel about those axes, altering the 

 latitudes of all places, and yet not in the least altering the position of the 

 axis of rotation among the stars. 



Perhaps by a more extensive search and analysis of the observations of 

 different observatories, the nature of the periodic variation of latitude, if it exist, 

 may be determined. I am not aware of any calculations having been made to 

 prove its non-existence, although, on dynamical grounds, we have every reason 

 to look for some very small variation having the periodic time of 325 '6 days 

 nearly, a period which is clearly distinguished from any other astronomical cycle, 

 and therefore easily recognised. 



