Letter | Published:

The Bifilar Pendulum at the Royal Observatory, Edinburgh

Nature volume 52, page 223 | Download Citation



SOME interesting readings of the bifilar pendulum, designed by Mr. Horace Darwin for measuring movements in the earth's surface, were made here at noon on the 9th inst. This instrument, which indicates oscillations in a north and south direction, was erected in March of last year, and daily observation of it has since been carried on, the scale being read off each minute, from five minutes before to five minutes after Paris noon. On the 9th inst. nothing unusual was noticed during the first seven readings, these being all practically the same; but on putting my eye to the telescope for the eighth, I at once noticed that during the interval of less than a minute since the preceding reading, the mirror had rotated considerably about its vertical axis, the normal having moved towards the north, the difference between the seventh and eighth readings being no less than 7.6 mm. of the scale. An immediate examination of the lamp-stand showed it to be perfectly firm. After the regular daily readings were completed, others were made at intervals of generally two minutes, for half an hour after Paris noon. These showed two quite conspicuous oscillations of the mirror during its return to its original position, which it reached about thirteen minutes after noon. It continued to move beyond this point towards the south, till at 0h. 31m. Paris mean time it was 4.1 mm. south of the point at which the scale was first read off. Later readings in the course of the day showed that it was still moving slowly to the south, but no further oscillations were recorded. In the evening, when the mirror appeared to have come to rest, the sensitiveness of the instrument was tested, and with this the column headed “Tilt of mirrorframe” in the following table has been computed. The positive sign indicates a tilt to the north.

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  1. Royal Observatory, Calton Hill, Edinburgh, June 20.



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