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SCIENCE. 



[N. S. Vol. XL No. 271. 



tigations had failed to reveal a variation 

 having that period, and astronomers had 

 fallen back to the dogma of invariability. 

 Theorists are now confronted vsrith the ne- 

 cessity of accounting for a period of about 

 428 days instead of 305. It is known that 

 any movements of the earth's crust and of 

 the water and air in response to a displace- 

 ment of the pole of figure from the pole of 

 rotation have the effect of lengthening 

 Euler's period. It has been shown that 

 any difference in the equatorial radii also 

 has a tendency to increase the theoretical 

 period to correspond more nearly with the 

 fact. The known atmospheric and oceanic 

 currents have also been appealed to for an 

 explanation. It has also been shown that 

 it is not impossible that such impulsive 

 forces as are concerned in earthquakes and 

 volcanoes may produce some of the effects 

 observed. The net result, however, of the 

 investigations of theorists has been a series 

 of partial explanations, no one of which 

 stands all the tests which have been applied 

 to it. Each fails, either by furnishing a 

 law of motion which differs essentially from 

 the observed facts or by being quantita- 

 tively inadequate. 



This was the situation when the Interna- 

 tional Geodetic Association formulated and 

 adopted a plan for determining the actual 

 motion of the pole during a series of years 

 with the highest attainable accuracy. The 

 observations, in conformity with the plan 

 devised by them, were actually begun in 

 the latter part of 1899, and it is proposed to 

 continue these observations uninterruptedly 

 for at least five years. The plan of opera- 

 tions is that the actual motion of the pole 

 shall be determined by simultaneous obser- 

 vations at four stations widely separated in 

 longitude, at each of which an instrument 

 of the highest precision is to be so used as 

 to guard, as fully as possible, against all 

 known sources of systematic error. The 

 details of the plan have been worked out 



very carefully, and it is admirable in every 

 respect. 



The four stations selected are Gaithers- 

 burg, in Maryland, in longitude + 77 ; 

 Ukiah, California, in longitude + 123; Mizu- 

 sawa, in Japan, in longitude + 219, and 

 Carloforte, on the small island of San Pietro, 

 just west of Sardinia, in longitude + 351. 

 One of these stations occurs in each of the 

 four quadrants of longitude, reckoned from 

 Greenwich. All are within three seconds 

 of the parallel of 39 ° 8' 10". The conditions 

 considered in selecting them, other than 

 those indicated above, were that there 

 should be nearly symmetrical conditions as 

 to the character of the surface northward 

 and southward of the station to avoid un- 

 symmetrical refraction ; that the hygienic, 

 social, and climatic conditions should be 

 such that the observer might remain healthy, 

 comfortable, and contented ; and finally in 

 Japan and at the European station the re- 

 gion was carefully studied with reference to 

 the probable frequency of earthquakes. 



Two extra volunteer stations have now 

 been added to these four, one at the Cin- 

 cinnati observatory, which happens to be 

 nearly on the parallel of the four selected 

 stations, and one at Tschardjiu, in Turk- 

 estan. 



The four principal observatories have 

 been constructed with the utmost care, 

 under specifications furnished by the Inter- 

 national Geodetic Association, specifications 

 designed to insure that observations shall 

 not be vitiated by local differences of tem- 

 perature. The observatory proper is sur- 

 rounded by lattice-work to protect it from 

 direct sunlight and the roof is double. 

 During observations the roof is rolled back 

 to leave an observing slit two meters wide. 

 The walls are so low that the telescope pro- 

 jects above the roof during observations. 

 This fact, together with the unusual width 

 of the observing slit, puts the instrument vir- 

 tually in the open air during observations. 



