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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 30 lbs., 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 of the 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 



