482 Proceedings of Uoycd Society of Edinburgh. [sess. 
call their attention for a few moments to the instrument with 
which this observation has been recorded at Blackford Hill, — the 
bifilar pendulum, as it is called. I do not intend to enter upon 
a detailed description of the instrument, because it has already 
been described in the Reports of the British Association for the 
Advancement of Science, and elsewhere. I may remind the mem- 
bers of the Society, however, that the essential feature of the 
instrument consists in the reflection of a ray of light from the 
surface of a concave mirror, suspended in a loop of fine silver wire 
from two points nearly vertically over one another. If the upper 
point of support be moved slightly in a direction perpendicular to 
the plane of the wire, the mirror will rotate around its vertical 
diameter, and the amount of the rotation, or the sensitiveness of 
the instrument for any given displacement of the upper point, 
depends on the closeness of that point to the vertical line passing 
through the lower one. 
At the Royal Observatory the instrument and its recording 
apparatus are so placed that the plane of the wire lies east and 
west when the reflected ray falls on the middle of the roll of 
photographic paper. The mirror is therefore set in motion by dis- 
placement of the upper support in a north and south direction, or 
by the north and south component of displacements in any other 
direction, and movements in the east and west direction do not 
affect it, except to alter its sensitiveness. With the object of 
securing that the mirror shall not be disturbed by the slight vibra- 
tions of short duration set up in the rock by concussions happening 
in its immediate vicinity, it is immersed in paraffin oil of the most 
transparent kind obtainable. It is thus sensitive only to tremors 
or undulatory movements of the rock of some duration. That this 
is so has, I think, been amply shown by observation. I have 
never, so far, been able to find on the photographs any effect 
arising from such causes as slamming of doors, or the fall of heavy 
weights within the building, or the explosions which are constantly 
taking place at a quarry on the south of the hill, though I have 
noted the times of many of these explosions, and examined the 
photographic record for the purpose of determining whether any 
effects were produced by them. On the other hand, observations 
made by Dr Copeland and myself at Calton Hill showed that the 
