352 Professor Arthur Schuster [May 18, 



the mountain chains must not be compared with a large weight resting 

 on an understructure, but rather with a hghter body partially im- 

 mersed in a heavier one. Mountains, according to this theory, float in 

 the body of the earth very much like icebergs float in water. The truth 

 of this theory can only be tested by accurate measurement of the gravi- 

 tational force from which information may be derived on the distribu- 

 tion of density in the earth's strata near the surface. On the whole, 

 the measurements so far available have confirmed Pratt's hypothesis. 



More recently, another problem has occupied the attention of the 

 International Geodetic Association, and owing to its immediate in- 

 terest, has absorbed the greater portion of its funds. The astronomi- 

 cal world was surprised by the announcement of Professor Chandler 

 that he was able to demonstrate from existing observations that the 

 earth's pole describes a closed curve taking about fourteen months to 

 complete a revolution. The possibility of a periodic shift of the 

 earth's axis was foreseen by Euler, who calculated the time of revolu- 

 tion to be ten months, but observations did not show a sensible period 

 of that duration. No one apparently before Chandler tried to see 

 whether another period l^eyond a small annual one existed. The dis- 

 crepancy between the calculated ten and the observed fourteen months 

 was cleared up by Professor Newcomb, who pointed out that Euler's 

 •calculation was based on the supposition that the earth is an absolutely 

 rigid body. Any yielding would increase the length of the period, in 

 fact the earth must be more rigid than steel in order that the period 

 should be as short as fourteen months. This shows how indirect in- 

 formation on the physical properties of the earth may be obtained 

 sometimes in an unexpected manner, the periodic revolution of the 

 pole leading to an estimate of the average rigidity of the interior of 

 the earth. The total displacement of the pole of the earth from 

 its average position is small, never amounting to more than eight 

 metres. The accuracy wdth which that displacement can be measured 

 is a testimony to the excellence of our astronomical observations. It 

 is a type of work in which co-operation is absolutely necessary. The 

 subject has received additional interest through the suggestion made 

 by Prof. Milne, in his recent Bakerian lecture, that seismic disturb- 

 ances may be caused by the changes in the position of the earth's axis. 

 Considering that the distortions in the earth are sufficient to increase 

 the periodic revolution of the pole from ten to fourteen months, this 

 suggestion is well worth investigation, and the 800/. per annum spent 

 by this country in support of the work of the Geodetic Association 

 will be well employed if it allows the vagaries of our pole to be more 

 closely studied and all the dimensional quantities of the surface of the 

 earth to become more accurately known. 



The contributions received by the Central Bureau of this Associa- 

 tion from the participating states amount to about 3000/., and there 

 is a balance which at the end of 1904 amounted to over 5000/. 

 The expenditure during 1005 was nearly 5000/., reducing the balance 



