ON THE MEASDEEMENT OF THE LUNAR DISTURBANCE OF aRAYITY. 119 



In our experiment of March 16 to 18, 1880, we found tliat the 

 pendulum stood furthest north about 6 p.m., so that at that time the S. 

 was most elevated ; and in the short series of observations during the 

 present summer the maximum elevation of the S. took place about noon. 



On October 1, 1878, M. Plantamour began a new series of observa- 

 tions, which lasted until September 30, 1879. The levels were arranged 

 in the two azimuths as before, and the observations were taken five times 

 a day, namely, at 9 a.m., noon, 3, 6, and 9 p.m. The mean of these five 

 readings he takes as the diurnal value. 



During October and November the eastern end of the level fell, which 

 is exactly the converse of what happened during the spring of the same 

 year ; he concludes that the eastern end falls when the external tempera- 

 ture falls. 



When a curve of the external temperature was placed parallel with 

 that for the level, it appeared that there was a parallelism between the 

 two, but the curve for the level lagged behind that for temperature by a 

 period of from one to four days. 



This parallelism was maintained until the end of June, 1879, when it 

 became disturbed. From then until the beginning of September the E. 

 rose, but in a much greater proportion than the rise of mean temperature. 

 It must be noted that July was a cold and wet month. 



Although the external temperature began to fall on August 5, the 

 E. end continued to rise until September 8. This he attributes to an 

 accumulation of heat in the soil. The total amplitude of the annual 

 oscillation from E. to W. amounted to 28"'08. 



There was also a diurnal oscillation in this azimuth which amounted 

 to 3""2 on September 5. The east end appeared to be highest between 6 

 and 7.45 p.m., and lowest at the similar hour in the mox'ning.' 



The meridional oscillations were much smaller, the total annual 

 amplitude being only 4"-89. From December 23, 1878, until the end of 

 April, 1879, there was a correspondence between the external temperature 

 curve and that for N. and S. level. We have already quoted the remark 

 on the diurnal meridional oscillations. 



M. Plantamour tells us that in 1856 Admiral Mouchez detected no 

 movement of the soil by means of the levels attached to astronomical 

 instruments. On the other hand, M. Hirsch established, by several years 

 of observation at Neuchatel, that there was an annual oscillation of 

 a transit instrument from E. to W., with an amplitude of 23", and an 

 azimuthal oscillation of 75", Similar observations with the transit 

 instrument were made at the observatory at Berne in the summer of 1879. 



It is to be regretted that M. Plantamour does not give us more 

 information concerning the manner in which the iron support for the 

 levels was protected from small changes of temperature, nor with regard 

 to the effect of the observer's weight on the floor of the room. We have 

 concluded that both these sources of distui'bance should be carefully 

 eliminated. 



' It seems that M. Plantamour sent a figure to the French Academy with the 

 paper, but no figure is given. This figure would doubtless have explained the mean- 

 ing of some passages which are somewhat obscure. Thus he speaks of the mitii- 

 mwn occurring between 6 and 7.45, but it is not clear whether minimum means E. 

 highest or E. lowest. Hinterpret the passage as above, because this was the state of 

 things in the observations recorded in the first of the two papers. There is a similar 

 difficulty about the meridional oscillatious. 



