ON EARTH TREMORS. 303 



tinuously for two hours and a half, no further oscillations were percep- 

 tible, and the usual steady movement was resumed. 



Another series of pulsations was observed on June 6 at 7.21 a.m., and 

 lasting nearly an hour, but I was unable at this time to watch them 

 carefully. From 7.0 to 7.8 the image was perfectly steady. At 7.21, 

 when I went down to the cellar again, and for about ten minutes after- 

 wards, it was performing oscillations even more extensive than on the 

 previous occasion, but it was again not possible to measure the exact 

 range owing to the variable limiting positions. Between 7h. 32m. 25s. 

 and 7h. 35m. 22s. there were nine complete oscillations, the average 

 length being 19" 7 seconds. At the end of this interval the range had 

 decidedly diminished. At 7.45, from my recollection of the limiting 

 positions of the image, I found the maximum range to be about 

 ^ second ; and I do not think this estimate can be much in error. By 

 7.55 the range had diminished to about -^ second (measured). At 8.7 

 the movement was almost imperceptible, and ten minutes later the image 

 was steady again. 



It is probable that these earth-pulsations were the dying-out vibrations 

 from some distant and severe earthquakes, but I have not been able to 

 identify them with any particular shocks. On May 23 a violent earth- 

 quake was felt at Thebes, in Greece, laying a great part of the city in 

 ruins, and it is stated that shocks were felt daily for some time afterwai'ds. 

 Some of these may possibly have produced the earth-pulsations observed 

 on June 3 and 6. 



Horizontal Pendulum of Dr. JE. von Rebeur-PascJiwitz. 



Dr. von B,ebeur-Paschwitz's horizontal pendulum is a modified form 

 of Professor Zollner's, which is described in the Report for 1881, pp. 

 112-114. The principal difference consists in the method of suspension 

 of the pendulum, the two stretched springs of Professor Zollner's instru- 

 ment being replaced by steel points working in agate cups, which allow 

 a nearly frictionless motion. In the following pages the pendulum used 

 by Dr. von Rebeur-Paschwitz is first described, and this is followed by 

 an account of the improvements suggested to him by the experience of 

 several years' work with the instrument.* 



The heavy cast-iron tripod-stand of the pendulum consists of a low 

 cylindrical vessel, on which, at equal distances apart, are three projec- 

 tions for the reception of the foot- screws, the whole being cast in one 

 piece. The cylinder is open at the top, and, after the instrument is 

 adjusted, is covered with a closely-fitting bell-glass. The screws have a 

 pitch of 0"36 mm., and are provided with large heads, so that a very small 

 change may be given by them to the level of the stand. The distance 

 between each pair of screws is 435 mm. Exactly half-way between two 

 of the foot-screws and opposite the third is an opening in the cylinder, 

 which is closed by a plano-concave lens, 75 mm. in diameter, and about 

 4"6 m. in focal length, the optical axis of the lens being horizontal and 

 directed towards the centre of the tripod-stand.^ 



' The pendulum is described in Dr. E. von Kebeur-Paschwitz's valuable memoir. 

 Das Horizontalpendel, &c., pp. 17-41. The suggested improvements are contained 

 in pp. 213-216 of the 8ame memoir, and also in letters to members of this Com- 

 mittee. 



* The lens is used for photographic registration ; for direct vision it is replaced 

 by a plane glast. 



