Two Unfelt Earthquakes

Abstract

REFERRING to Prof. John Milne's interesting communication on “Two Unfelt Earthquakes,” asking for information as to whether these disturbances have been instrumentally recorded elsewhere, Dr. Copeland requests me to say that an examination of the photographs of the oscillation-curve of the bifilar pendulum at this observatory shows several disturbances on February 7, the first of the dates mentioned by Prof. Milne. These are as follows:—At 7.37 a.m. an abrupt movement of the pendulum towards the north; from 8.24 to 8.41 a.m. a distinct reduction in the intensity of the colour of the photographic trace, doubtless due to tremor of the mirror, and, though not so well marked, yet quite similar in character to the gaps described in a previous letter (NATURE, No. 1410, November 5, 1896); at 5.32 p.m. another abrupt movement to the north, and a similar one at 1h. 20m. after midnight. The three abrupt movements have been compared with the mean of two measures of the sensitiveness of the instrument, and show tilts in the mirror frame of 1˙6, 1˙1, and 1˙3 seconds of are respectively. In each case the mirror returned to the normal position slowly after a period of from 2 to 4 hours.