190 Mr. C. Davison on the Periodic 



astronomers and physicists in different parts of the world. 

 But, at Sfecheron, near Geneva, they have been conducted by 

 M. Philippe Plantamour with an energy and perseverance 

 which can find but few parallels, even in the annals of science. 

 Since the end of 1878, without any important intermission, 

 have his levels been read at least twice a day, and the results 

 described in a series of valuable memoirs, published annually 

 in the Archives des Sciences physiques et naturelles *. 



In this paper I propose to summarize the conclusions at 

 which M. Plantamour has arrived : to show that a slightly 

 different discussion of his observations will lead to a result of 

 some interest ; and to consider the cause or causes to which 

 these movements may be due. 



Arrangement of Levels, 8fc. — M. Plantamour^s levels are 

 placed in the cellar of his house at Secheron, nearly two kilo- 

 metres north of the observatory of Geneva, and 27 metres 

 distant from the lake. The east front of the house faces the 

 lake and is parallel to its shore-line, its direction being 30° E. 

 of N. to 30° W. of S. The cellar^ which is very dry, is 

 excavated in compact blue glacial clay, covered by gravel on 

 the west, north, and south sides of the house ; and the ground 

 on the west side is thus 2 metres higher than that on the 

 east. M. Plantamour says that the subsoil of the house is 20 

 centimetres lower than the terrace; that is, I suppose, the 

 clay beneath the floor of the cellar is at this depth below the 

 ground on the east side of the house. Between the house 

 and the lake the ground is nearly horizontal, the level of the 

 lake being on an average one metre below the terrace. 

 Behind the house the ground rises at an inclination of about 

 3 or 4 in 100 for about a kilometre, beyond which the slope 

 increases more rapidly. In the immediate neighbourhood of 

 the house, however, the slope of the land is considerable, and 

 its direction cannot differ greatly from 30° 8. of E. I give 

 these details somewhat fully, as they have an important 

 bearing on the results f. 



* *' Sur les d^placements de la buUe des niveaux a bulle d'air," Arch, 

 des Sc. (July 1878) nouv. per. vol. Ixiii. pp. 5-12; "Des mouvements 

 p6riodiques du sol accuses par des niveaux a bulle d'air," Arch, des Sc, 

 3'"' per. (Dec. 1879), vol. ii. pp. 641-662 ; (Feb. 1881) vol. v. pp. 97-106: 

 (Dec. 1881) vol. vi. pp. 601-610; (Dec. 1882) vol. viii. pp. 551-558; (Dec. 

 1883) vol. X. pp. 616-622 ; (Nov. 1884) vol. xii. pp. 388-394 ; (Nov. 1885) 

 vol. xiv. pp. 443-448; (Dec. 1886) vol. xvi. pp. 566-571. 



t The above account is taken partly from M. Plantamour's papers, 

 partly from a letter to myself in which he describes the conditions at 

 greater length. In a MS. sketch of the plan of his house, which he was 

 good enough to send me, it would appear that the direction of the front 

 of his house and of the shore-line of the lake is 17^° E. of N. to 17|° W. 

 of S. : but I am not certain whether this plan is drawn exactly to scale; 



