INTERNATIONAL SCIENCE. 503 



wonder how mountain chains such as the Himalayas could rest on the 

 lower strata of the earth without crushing them and forcing them in 

 by the pure power of their weight, and the most plausible theory to 

 account for this was found in the idea first suggested by Pratt that 

 tlie mountain chains must not be compared with a large Aveight rest- 

 ing on an understructure, but rather with a lighter body partially 

 immersed 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 gravitational force from which information may be derived on 

 the distribution 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 inter- 

 est has absorbed the greater portion of its funds. The astronomical 

 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 com- 

 plate a revolution. The possibility of a periodic shift of the earth's 

 axis was foreseen by Euler, who calculated the time of revolution 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 beyond a small annual one existed. The discrepancy 

 between the calculated ten and the observed fourteen months was 

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

 culation 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 

 information on the physical i)roperties 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 

 aA'erage position is small, never amounting to more than 8 meters. 

 The accuracy with which that displacement can be measured is a 

 testimony to the excellence of our astronomical observations. It is 

 a type of work in which cooperation is absolutely necessary. The 

 subject has received additional interest through the suggestion made 

 by Professor Milne, in his recent Bakerian lecture, that seismic dis- 

 turbances 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 £300 per 

 amium spent by this country in support of the work of the geodetic 



