ASTRONOMICAL PROBLEMS CURTIS. 335 



logical explanation for this small shift of the plane of the Equator. 

 The accumulation of snow and ice at one pole, together with the cor- 

 responding diminution at the other pole, due to the melting in the 

 summer season, would be perhaps sufficient to explain the shift, but 

 if this were the true and only explanation, it is difficult to see why 

 the maxima and minima do not follow the solstitial points by a con- 

 siderable interval of "lag," instead of preceding them by about 10 

 days. 1 Moreover, the quantities involved are so extremely minute, 

 such transcendental care is necessary in arranging and making the 

 observations, and such pains to exclude in the investigation all pos- 

 sible sources of systematic error, that astronomers are by no means 

 in accord as to the real existence of the s-term. Biske has shown that 

 a variation similar to that afforded by the s-term could arise as a 

 result of inaccuracies in the adopted value of the solar nutation, and 

 that future progressive changes in this value could result from 

 similar slight errors in the adopted value of the lunar nutation. 

 Quite recently Prof. Hiroyama, of Tokio, has subjected the results 

 of the first four years of the latitude variation results to a careful 

 analysis, and reached the conclusion that the s-term is probably a 

 result of errors which may be classified as instrumental. He did 

 not include in his researches, however, the results from the southern 

 stations. 



Probably no more marked case of modern specialization in the 

 science of astronomy, no more fitting example of minute and careful 

 analysis, nor any better illustration of the mutual interdependence 

 of fields of investigation apparently widely separated, can be found 

 than this same subject of the variation of latitude. Long since, 

 Euler, from purely mathematical considerations with regard to a 

 rotating spheroid, showed that the axis of the earth should be sub- 

 ject to a minute oscillation, with a period of 305 days. In 1890-91 

 Prof. Kiistner announced that this prediction had been confirmed by 

 observation, but that the period was about 427 days. So minute is 

 the movement that the poles shift from their mean position by less 

 than 30 feet. Eight special observatories have been established, six 

 in the Northern Hemisphere and two in the Southern, and experi- 

 enced observers are carefully accumulating the observations for the 

 further study of this variation, determining from observations of 

 the stars a periodic movement of the positions of the poles of the 

 earth only a little greater than the distance from one wall of their 

 small observatories to the other, and even showing, with some prob- 

 ability, that the earth's center of gravity oscillates once a year a dis- 

 tance of only a little over 4 feet toward one pole or the other. From 

 these results Darwin, Hough, Larmor. and others have undertaken 



3 Later studies seem to indicate that the maxima and minima in the z-term are slowly 

 shifting. 



