478 ' REPORT— 1894. 



zenith of the observer's station ; in other words, by repeated determinations 

 of geographical latitudes. But, notwithstanding very long and refined 

 determinations of the geographical latitudes at some of the principal 

 observatories, beginning shortly before the middle of the present century, 

 only very uncertain and discordant traces of the phenomena in question 

 were found. 



The reason for this want of success is now very clear. Astronomers 

 had limited their researches too narrowly to the last-mentioned type — 

 namely, to the supposed regular ten-monthly periodical movement of the 

 pole of the rotational axis around the pole of the principal axis. Too 

 easily it had been admitted that all the existing variations of the distri- 

 bution of terrestrial masses were far too small for altering sensibly the 

 position of this principal axis itself. 



It was Lord Kelvin, at the Glasgow meeting of the British Association 

 (1876), who first drew the attention of the scientific world to the con- 

 sideration of the great natural transports of masses of air and water, and 

 of various masses by the water, going on continuously and periodically in 

 the form of currents and circulations of different kinds, as well in the 

 atmosphere as in oceans and I'ivers, and the effects of the enormous 

 periodical deposits of snow and ice. He showed that these very consider- 

 able variations of the distribution of masses on the earth could not only 

 produce sensible displacements of the principal axis of inertia, but that 

 such displacements of this axis could have an amplifying effect on the 

 total amount of displacements of the x'otational axis. 



For if the principal axis were itself not in a constant position, the 

 theoretically required movement of the rotational axis around the principal 

 axis would become a very complicated movement, differing entirely from 

 the simple form which to that time had appeared in the researches of 

 astronomers. 



This epicyclic character of the movement of the pole of the rotational 

 axis could considerably modify not only the length of the period, but also 

 the whole geometrical character and amplitude of the curve, in such a way 

 that in longer periods epochs of very small variations of latitude could 

 alternate with epochs of considerably increased variations of latitude. 

 Possibly, as a further consequence of this complication of the displacements 

 of the two axes, and as a consequence of the still existing plastic state of 

 certain parts of the earth, as well as by the damping effects of the fluid 

 parts, even ])rogressive — though very slow and unsteady progressive — 

 displacements of the rotational axis in the earth could still result. 



The field of this research was thus decisively cleared by Lord Kelvin ; 

 and finally, about four years ago, by the co-operation of some Observatories 

 with the International Geodetic Union, clear evidence was obtained, and 

 in the last three years, with the aid of an expedition sent by the Inter- 

 national Union to Honolulu, decisive proofs of such displacements have 

 been found. I consider it a special honour and pleasure to be enabled to 

 submit some of the newest results of this international co-operation to a 

 meeting of the same Association which, twenty years ago, had been witness 

 of the almost prophetic assertions of one of its most illustrious members 

 regarding the real conditions of this important phenomenon. 



I have prepared a diagram showing these newest results. In this 

 diagram is given a representation of the wanderings of the pole of tlie 

 rotational axis of the earth on its surface during the last twenty months, 

 from October 20, 1892, to May 1, 1894. 



