ON EARTH TREMORS. 



157 



The movements of the pendulum during a storm on May 4, 1893, are 

 represented in fig. 5 (1 mm.s=0"-044). 



Fig. 5. — Movements during a Storm on May 4, 1893. 



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lliKMIM'lII^'Q'Wtrt*^'**"^ 



Lastly ti«-. 6 (1 mm.=0"-025) serves to illustrate the deviations (5), 

 when the' pendulum, without being agitated, is never at rest, but for 

 several hours inclines sometimes in one direction, sometimes in the other. 

 During the month of March, 1894, such disturbances frequently occurred. 



Fig. 6.— Disturbances of Short Period (probably seismic) on March 5, 1894 



On the whole, the observations of the horizontal pendulum may be of 

 much service in studying the different movements of the earth's crust and 

 of the ground. 



Amongst other things it seems to me difficult to explain the oscilla- 

 tions of the diurnal period observed here, as well as at Potsdam, &c., by 

 M. von Rebeur-Paschwitz, and at Charkow by Professor Lewitzky, since 

 the temperature and relative humidity of the air in the neighbourhood of 

 the instrument remain constant throughout the day. 



Havin«^ at my disposal an almost uninterrupted series of observations 

 for fifteen months in the same position of the instrument (the axis of the 

 pendulum in the prime vertical), I wished to investigate if the moon 

 produced any influence on these oscillations. For this purpose I divided 

 the whole series into sixty successive groups, corresponding to the different 

 phases of the moon, from which I have drawn the conclusion that the 

 influence of the moon is insensible, or, if it exists, that it is masked by 

 the different accidental disturbances. After this, having divided the 

 whole series into five consecutive parts (three lunar months in each), I 

 have obtained the following table of the deviations of the pendulum from 

 its mean position for every two hours (astronomical time) in thousandths 

 of a second ' (0"-001) :— 



' A - sign denotes a deviation to the north, a + sign to the south. 



