563 Royal Institution. 



coexistence of t«o 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The experiment originally made by M. Foucault was repeated and 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 

 pui-sued 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 

 watched through a complete revolution, the time of which was ob- 

 served to be 28^ 26™. 



The sources of probable error are numerous and not easy to be 

 effectually guarded against. The most formidable perhaps is the 

 extreme difficulty of causing the pendulum to vibrate truly in one 

 plane, and to prevent its motion in a narrow ellipse. When this 

 takes place, and the arc is considerable, the direction of the major 

 axis is continually changing, owing to a well-known mechanical 

 cause (see Herschel's Outlines of Astronomy, p. 44-4) ; but this de- 

 viation is always in the same direction as that of the original motion 

 of the pendulum, and consequently changes when that direction is 

 changed. The true deviation may be distinguished from this, in 

 that it is always from east to west, independently of the direction of 

 the original impulse ; and the ball always passes accurately through 

 the centre in every oscillation, whereas in the former case it never 

 does. 



For great accuracy, a variety of other precautions are requisite, as 

 to the perfect freedom of suspension, guarding against currents, &c. ; 

 it is, however, possible that the elliptic deviation may oppose that 

 due to the earth's rotation, while the latter may manifest itself in 

 spite of the former. 



It is extremely probable that many of the public repetitions may 

 have been affected by these causes of error ; yet some of those re- 

 ferred to have been made by men of so much eminence and expe- 

 rience as observers, as to render it highly improbable that they should 

 not have been sufficiently guarded against every source of fallacy. 

 The accordance of many of the results at different places within fair 

 limits of error, is also a strong argument in favour of their accuracy 

 and trustworthiness. 



The rates of deviation for one hour as determined at different 



I 



