Bicrbank — Apparent Vai'iations of the Vertical. 323 



Art. XXXVI. — Some Apparent Variations of the Vertical 

 observed at the Cheltenham Magnetic Ohservatory • * by 

 John E. Bukbank. 



This paper deals -with some slow period cliano;es of level of 

 the piers on which the Oinori horizontal peiidnluni seismo- 

 graph has been mounted at the Cheltenham Magnetic Obser- 

 vatory. 



From November, 1904, to October, 1907, both components 

 were mounted on stone piers in the north room of the vari- 

 ation observatory. f In October, 1907, the entire instrument 

 was moved to a new location and mounted on a massive con- 

 crete pier. % A comparison of the diurnal oscillation of level 

 and also the changes of level due to temperature variations, in 

 the two places, shows that the magnitude and direction of 

 these effects are to a large extent local. 



The diurnal oscillation of level has been noted at many seis- 

 mological stations. Professor J. Milne § has studied this effect 

 in Japan and elsewhere and has observed it in the soil 12 feet 

 below ground level, but did not observe it when the instrument 

 was mounted on the solid rock in a cave. He found the diur- 

 nal wave hardly appreciable in a wood where the trees protected 

 the pendulum house from the direct effects of the sun. At all 

 stations where the ground was covered with trees or buildings 

 more upon one side of the station than upon the other, the 

 diurnal waves were large, and often differed in phase. At 

 most stations on wet and cloudy days diurnal waves were 

 absent. 



E. von Rebeur-Paschwitz II compared the diurnal oscillation 

 of the surface of the ground at three places — Wilhelmshaven^ 

 Potsdam, and Puerto Orotavo, Teneriffe ; he found the effect 

 most pronounced in the east-west direction, and comparatively 

 small in the north-south direction. 



At Wilhelmshaven the pier was erected in the cellar of the 

 Naval Observatory, the surface of the ground is marshy, with 

 a layer of 2 to 3 meters of clay, and below this sand to a great 

 depth. 



At Potsdam the pier was mounted in the cellar below the 



* Communicated by permission of the Superintendent U. S. Coast and 

 Geodetic Survey. Read before the Philosophical Society of Washington, 

 Jan. 29, 1910. 



■)• For description of the Observatory see Appendix No. 5, Report of the 

 Superintendent U. S. Coast and Geodetic Survey, 1902 ; also Terrestrial 

 Magnetism, vol. viii, pp. 11-30, March, 1903. 



t See Terrestrial Magnetism, vol. xiii, pp. 1-20, March, 1908. 



§ J. Milne, Movements of the Earth's Crust, Geographical Joiirnal, vol. vii, 

 p. 229, March, 1896. 



II See reprint in British Association Report, 1893. 



