ON EARTH TREMORS. 155 



APPENDIX I. 



Account of Observations made with the Horizontal Pendulum at Nicolaiew.^ 



By Professor S. Kortazzi. 



More than a year has elapsed since I began to make observations 

 with the pendulum of M. E. von Rebeur-Paschwitz installed in the cellar 

 of the observatory, but I have not as yet published any detailed reports on 

 this subject, with the exception of a brief account given at one of the 

 meetings of the Astronomical Society at St. Petersburg. . . . The more 

 or less regular oscillations of the pendulum, as well as the abrupt per- 

 turbations which it often experiences, depend on several physical agents, 

 and I find it necessary to continue the series of observations for several 

 months to be in a position to obtain from them more or less sound results. 



For the present I can only draw some general conclusions : — 



(1) The horizontal pendulum may be used as a very sensible and very 

 trustworthy seismograph, which does not fail to record all the oscillations 

 of the ground and tremors of the earth's crust, even in the case of very 

 distant earthquakes. Comparing the results obtained here with those at 

 Strassburg during the first three months of this year, we find more than 

 a dozen disturbances registered at the same time by both instruments. 



(2) Different seismic disturbances produce extremely varied move- 

 ments of the pendulum. On the enclosed copy of the photograph, which 

 registers the positions from April 3, 8h., to April 9, 7h., are shown two 

 feeble disturbances on April 4 at 22h. 40m. and April 6, 5-5h., and a 

 very strong one on April 8 at 4-Oh. The latter corresponds to the earth- 

 quake wliich took place at this time in Servia and Southern Hungary. 

 For three-quarters of an hour the pendulum was very strongly disturbed ; 

 it even changed abruptly its normal position ; and it was only at 6h. that 

 it became steady, whilst no one in the whole country felt then the least 

 movement of the ground. On August 17, however, at 4|h. mean time, 

 a rather pronounced eartliquake occurred at Nicolaiew itself, and was 

 observed by a great number of the inhabitants, whilst the pendulum only 

 experienced a feeble disturbance similar to that shown on the curve on 

 April 6 at 10-5h. (but much more feeble), when the pendulum was 

 purposely disturbed by a feeble current of air under its cover. 



(3) The pendulum is subject to periodic diurnal and annual oscilla- 

 tions. The amplitude of the former does not on an average exceed 0"-l, 

 whilst that of the latter attains 3" or 4". These last changes may be 

 explained by the inclination of the upper layers of the ground produced 

 by the annual changes of temperature at the depth of the pillar ; whilst 

 the diurnal oscillations, it seems to me, cannot be explained in the same 

 way, because not only the ground at the depth of 15 feet, at which the 

 pillar of the pendulum is founded, but even the air of the cellar, does not 



' Communicated in two letters (dated August 31, 1893, and July 10, 1894) to the 

 Secretary, the second being an abstract of a report to be presented to the Societe 

 AstroDomique Russe. 



