824 Biu'bank — Apparent Variations of the Vertical. 



east tower of tlie Astrophysical Observatory, the soil being 

 sand to a great depth. 



At Puerto Orotavo the seisinograpli was mounted at ground 

 level on the cement floor of a small building on the side of 

 Pico de Teyde about 18 kilometers from the summit. 



The diurnal range of oscillation of the plumbliue (apparent 

 variation of the vertical) at these three ])laces agrees remark- 

 al)ly well with the ranges of those elements which are a meas- 

 ure of the intensity of the solar radiations ; the actual range 

 ditifering, however, between tlie three places. Tlie times of 

 maximum east and west displacements also differ. 



In fig. 2 are given curves for the diurnal oscillation of the 

 plumbliue on clear days for the three places : (()) Puerto 

 Orotavo, (P) Potsdam, and (W) Wilhelmshaven. Negative 

 ordinates indicate motion of plumbliue toward east, and positive 

 ordinates motion toward west. An increase of temperature 

 caused deviation toward the east in each ease, the amount at 

 Wilhelmshaven being far the largest of the three. Barometric 

 variations caused tilting at Wilhelmshaven and Puerto Orotavo 

 but not appreciable at Potsdam. 



Dr. J. R. Sutton* found that the seismograph piers at Kim- 

 berley, S. Africa, showed a diurnal variation of level with a 

 range of from about 2 seconds of arc in summer to about 

 4 seconds in winter. The maximum westerly elongation 

 occurred about 5h. 30m. a.m., and the maximum easterly about 

 4h. 15m., the median positions a little before 11 a.m. and 9h. 

 30m. p. M. 



Mr. B. M. Varneyt studied the diurnal variation at Cam- 

 bridge, Mass., and found that it occurred only on days when 

 the sun shone. The E-W component showed a tilting toward 

 the east during the forenoon, which died out about noon and 

 was followed by a tilting toward the west during the afternoon. 

 The N-S component showed a tilting toward the south during 

 the forenoon, and to the north later in the day. It was much 

 smaller than the E-W motion. 



The existence of this diurnal oscillation of the level has been 

 noted by Ehlert,:}: at Strassl)urg, at a depth of 5 meters below 

 the surface; by Denison,§ at Victoria, B.C.; Lagrange, at 

 TJcclell ; Claxton, at Mauritius,^!' and Wallis,** on the Milne 



* Preliminary Note on the Diurnal Variation of Level at Kiinberley, Trans- 

 actions Eoyal Society of S. Africa, vol. i, pt. 1, pp. 803-309. 



■j- Some Long-period Deviations of the Horizontal Pendulum at the Harvard 

 Seismographic Station, Science, Feb. 11, 1910. 



X Beitrage zur Geoijhysik, iv, p. 70. 



§ The Effect of Atmospheric Pressure upon the Earth's Surface, Journal 

 of the Royal Astronomical Society of Canada. Nov. -Dec, 1908. 



I Periodicity Sismique Bulletin de la Soeieti^ Beige d'Astronomie, July, 1904. 



i[ Results of Mag. and Met. Observations made at Mauritius in 1906. 



** Not yet published. 



