118 REPORT— 1881. 



He does not give any theoretical views as to the causes of these 

 phenomena, but remarks that his observations tend to prove that the 

 causes of change are sometimes neither astronomical nor thermometrical. 



The most sudden change which he noted was on October 27, 1872, 

 when the vertical changed by 2"'4 in six hours and a quarter. Between 

 January 30 and March 26 of the same year the plumb-line deviated 4"'5 

 towards the south. 



We now come to the valuable observations of M. Plantamour, which 

 we believe are still being prosecuted by him. His papers are ' Sur le 

 deplacement de la bulle des niveaux a bulle d'air,' ' Comptes Rendus,' 

 June 24, 1878, vol. 86, p. 1522, and ' Des mouvements periodiques du 

 sol accuses par des niveaux a bulle d'air,' 'Comptes Rendus,' December 1, 

 1879, vol. 89, p. 937. 



The observations were made at Secheron, near Geneva, at first at the 

 Observatory, and afterwards at M. Plantamour's house. After some 

 preliminary observations he obtained a very sensitive level and laid it on 

 the concrete floor of a room in which the variations of temperature were 

 very small. The azimuth of the level was E. and W., and the observa- 

 tions were made every hour from 9 a.m. until midnight. Figures are 

 given of the displacement of the bubble during April 24, 25, and 26, 1878. 

 The results indicate a diurnal oscillation of level, the E. end of the level 

 being highest towards 5.30 p.m. ; the amplitudes of the oscillations were 

 8""4, ll"-2, 15"'75 during these three days. It also appeared that there 

 was a gradual rising of the mean diurnal position of the E. end during 

 the same time. 



The level was then transported to a cellar in M. Plantamour's 

 house, when the temperature only varied by half a degree centigrade. 

 The bubble of the level often ran quite up to one end. A new and larger 

 level was obtained, together with the great 'chevalet de fer,' which is 

 used by the manufacturers in testing levels. Both levels were placed E. 

 and W., at about two meters apart. During May 3 and 4, 1878, the 

 bubble travelled eastward without much return, and it is interesting to 

 learn that simultaneous observations by M. Turretini, at the Level 

 Factory, three kilometers distant, at Plainpalais, showed a similar 

 change. 



Between May 3 and 6 the level actually changed through 17". Up 

 to the 19th the level still showed the eastward change. 



M. Plantamour remarks that the eastern pier of a transit instrument 

 is known to rise during a part of the year, but not by an amount com- 

 parable with that observed by him, and that the diurnal variations are 

 unknown. 



After further observations of a similar kind one of the levels waa 

 arranged in the N. and S. azimuth. 



The same sort of diurnal oscillations, although more irregular, were 

 observed, but the hours of maximum were not the same in the two levels. 

 During the four days May 24 to 28 the maximum rising of the north 

 generally took place about noon. This is exactly the converse of what 

 we have recently observed. 



In the second paper he remarks : 



* Dans le sens du meridien, les mouvements diurnes sont tres rares 

 irreguliers et tou jours tres faibles, le niveau en accuse parfois, quand il 

 n'y en a point de Test a I'ouest, et inversement, quand ces derniers sont 

 tres prononces, on n'en aper9oit que tres rarement du sud au nord.' 



