1851.) OF THE ROYAL INSTITUTION. 71 
result would however hardly amount to any more palpable proof to 
the senses than other astronomical phenomena afford ; in this case, 
as well as in those, the conclusion is equally derived from reasoning 
on the actual appearances. 
An idea of such an effect seems to have occurred long ago, 
and is mentioned in a paper in the Phil. Trans. 1742, No. 468, by 
the Marquis de Poli, in the course of some observations on the 
pendulum of a different kind. He remarks, ‘I then considered 
(adopting the hypothesis of the Earth’s motion) that in one oscilla- 
tion of the pendulum there would not be described from its centre 
perfectly one and the same arc in the same plane :’”— but he does 
not pursue the subject, as being foreign to his immediate object. 
It appears also (see Comptes Rendus, 1851, No. 6) that in 1837 
Poisson had hinted at such an effect, but supposed it of insensible 
amount. 
To some minds difficulties present themselves in the first instance, 
which are easily removed by a few simple illustrations. In the first 
place the deviation from parallelism to itself, of the meridian of any 
place, during the rotation of the Earth, is a simple geometrical question 
easily determined, and the inclination expressed by a trigonometrical 
formula. In the next place the independence of the motion of the 
pendulum, notwithstanding that the point of support is carried along 
with the earth in its rotation, and that the whole seems to form a 
part of the earth, is a point easily elucidated by very simple experi- 
ments, in which the vibration of a small pendulum is seen to continue 
parallel to itself notwithstanding a motion given to the point of 
support ; the effect being in fact only a simple consequence of the 
coexistence of two motions communicated to a body at the same 
time. A beautiful apparatus, lent by Mr Bishop for showing this, was 
exhibited on the present occasion. 
The experiment originally made by M. Foucault was repeated and 
confirmed under the inspection of M. Arago, and other eminent 
scientific men with all due precautions in Paris, as also at Ghent, 
Brussels, and elsewhere. In England besides the public repetitions 
at the Russell, London, and Polytechnic Institutions, by Dr. Roget, 
Mr. Bishop, and Mr. Bass, the experiment has been tried at York 
by Professor Phillips, and at Bristol by Mr. Bunt, with careful atten- 
tion to all the circumstances likely to ensure the avoidance of sources 
of error, and to secure precise results. At the Royal Institution 
on the present occasion the experiment was exhibited under two 
modifications by Dr. Bence Jones and by Mr. Bass. Other observers 
have also repeated it in various places, especially at Dublin, where 
Messrs. Haughton and Galbraith, Fellows of Trinity College, have 
pursued the research with all imaginable precautions, and have 
obtained results somewhat different from those of other observers. 
According to nearly all the other experiments the rate of deviation 
continued uniform: according to Messrs, Haughton and Galbraith, 
it varied: and they seem to have been the only observers who have 
