Hurhanh — Apparent Variations of the Vertical. 325 



seismograph, at Honolulu Magnetic Observatory. It has also 

 been noted at Sitka, Alaska, at the Weather Biii-eau in Wasli- 

 ington, and doubtless at nearly every seisinologieal station in 

 the world.* Dr. Heckerf states that it was not entirely elimi- 

 nated at a depth of 25 meters below the surface at Potsdam. 



This diurnal oscillation of the surface of the ground appears 

 to be a general efi'ect which does not admit of easy explanation. 

 It appears, however, in some cases to be largely conditioned by 

 the immediate surroundings of tlie pier. 



The seasonal and annual variation of level has been studied 

 by Paschwitz, Denison, Wallis, Hecker, Klotzij: and numerous 

 others. The results for the stations on the continental areas 

 appear irregular and are not easily explained. 



An attempt to determine the annual range of level at this 

 observatory shows that it is comparatively large, but the results 

 are far from satisfactory. The screw which controls the lateral 

 movement of the cylinder, spacing successive lines 3 millimeters 

 apart, does not have uniform pitch and successive days' records 

 are not always controlled by the same part of the screw. This 

 error of the screw prevents a study of the long-period varia- 

 tions of level, but is not great enough to seriously affect results 

 extending over a period of one or two weeks. 



The possible effect of variations of atmospheric pressure on 

 the change of level has been studied by von Rebeur Paschwitz,§ 

 who noted at Wilhelmshaven a deviation of level of 0'29" of 

 arc for one millimeter variation of the barometer. This extraor- 

 dinary fluctuation was attributed to the marshy, spongy nature 

 of the ground. His later observations at Strassburg show that 

 the efi'ect of barometric variations there is uncertain. 



Professor Omori describes three cases of remarkable tilting 

 of the ground in Japan during storms. At Tokyo, on Oct. 

 10-11, 1904,11 when an extreme oscillation of level amounting 

 to 3-5 seconds of arc occurred in a few hours, and again on 

 Jan. 10-11, 1906,^ when tlie extreme range amounted to 

 2"87 seconds of are, and at Mito, some distance northeast of 

 Tokyo on March 23, 1907,** when a range of 3-7 seconds of 



* The references in connection with tliis paper are very incomplete, as 

 they include only the few which have come to my attention. 



f Beobachtungen an Horizontalpendeln iiber die Deformation des Erd- 

 korpers unter dem Einfluss von Sonne und Mond, Verofif. des Kgl. Preussi- 

 schen Geodatischen Institutes, Neue Folge Nr. 32, 1907, 95 pages. 



X Report Chief Astronomer (Canada), 1907. Appendix 2, Ottawa, 1908. 



S Bertrage zur Geophysik, xi, p. 334. 



II Publications of the Earthquake Investigation Committee, No. 21, 

 pp. 5-8. 



T[ Bulletin Imperial Earthquake Investigation Committee, vol. i, No. 4, 

 August. 1907. 



** Bulletin Imperial Earthquake Investigation Committee, vol. ii. No. 1, 

 March, 1908. 



