118 
comotive Department of the Dublin and Kingstown Railway, 
and himself, to determine the Azimuthal Motion of the Plane 
of Vibration of a freely suspended Pendulum. 
« The experiments, of which the following is a brief ac- 
count, were made at the engine-factory of the Dublin and 
Kingstown Railway Company, with different modes of sus- 
pension, upon a pendulum 35 feet 5 inches in length, the 
bob of which was of iron made spherical in a lathe, with a 
point turned true, projecting from its lower surface: its 
weight being 30Ibs., and the pendulum being set in motion 
in the plane of the meridian. After some unsuccessful trials 
of various modes of suspension, we adopted one, which ap- 
pears liable to less theoretical objection than any other we 
could undertake at a short notice. It consisted simply of a 
number of parallel fibres of hemp or cocoon silk, drawn tightly 
through a small hole in a thick metallic plate, at the upper 
surface of which the fibres were secured. This plate was then 
screwed down upon a metal surface, accurately planed and 
levelled, with a large circular aperture to allow the string of 
the pendulum to play freely. The silk or hemp fibres were 
continued for about ten inches below the under surface of the 
plate, and the remainder of the string of the pendulum was 
composed of copper or pianoforte steel wire. Underneath the 
point of suspension we placed a horizontal table furnished with 
graduated circles, round the common centre of which travelled 
a moveable arm, divided into tenths of an inch by parallel 
lines. We were enabled by this simple contrivance to read off 
the azimuth of the plane of vibration from the opposite sides 
of the circle, and at the same time to measure with precision 
the magnitude ofthe minor axis of the small elliptic vibration — 
which accompanies the movement. 
‘¢ In order to try the mode of suspension used by M. Fou- 
cault himself, we replaced the hemp fibres by a pianoforte 
wire drawn tightly through a hole carefully drilled in a thick 
