"At the centre of the dome of the Panthéon a fine wire is attached, from which a sphere of metal, four or five inches in diameter, is suspended so as to hang near the floor of the building. This apparatus is put in vibration after the manner of a pendulum. Under and concentrical with it is placed a circular table, some twenty feet in diameter, the circumference of which is divided into degrees, minutes, &c., and the divisions numbered. Now, supposing the earth to have the diurnal motion imputed to it, and which explains the phenomena of day and night, the plane in which this pendulum vibrates will not be affected by this motion, but the table, over which the pendulum is suspended, will continually change its position, in virtue of the diurnal motion, so as to make a complete revolution round its centre. Since, then, the table thus revolves, and the pendulum which vibrates over it does not revolve, the consequence is, that a line traced upon the table by a point projecting from the bottom of the ball will change its direction relatively to the table from minute to minute and from hour to hour, so that if such point were a pencil, and that paper were spread upon the table, the course formed by this pencil would form a system of lines radiating from the centre of the table. The practised eye of a correct observer, especially if aided by a proper optical instrument, may actually see the motion which the table has in common with the earth, under the pendulum between two successive vibrations. It is, in fact, apparent that the ball, or rather the point attached to the bottom of the ball, does not return precisely to the same point of the circumference of the table after two successive vibrations. Thus is rendered visible the motion which the table has in common with the earth."

"The experiments connected with the practical demonstration of the phenomenon require to be conducted with great care; and some discredit has been brought upon attempts to illustrate it in England by persons who have not taken the necessary precautions to protect the motion from the excentric deviation to which it is liable, and which may, and indeed must, have the effect of causing, in some cases, an apparent failure, and in others a still more unfortunate, because fallacious, success. I believe, from the character of the persons connected with the experiments, that the true phenomenon has been accurately produced and observed in Paris. I doubt whether as much can be said, with entire confidence, of any of the experiments hitherto performed here in London.

"Any want of symmetry in the arrangements for the suspension of the wire, or in the centering of the weight, exposure to currents of air, or the tremulous motion occasioned by the passage of vehicles, may operate to cause a phenomenon to be brought about curious enough in itself, as a result of mathematical laws, but quite different from that supposed. The phenomenon of the progression of the apsides of an oval orbit, which is here alluded to, is familiar to all students in mechanics.

"It is perfectly absurd for persons unacquainted with mechanical and geometrical science to presume to make the experiment. Indeed, such efforts deserve rather the name of conjuring than of experiment; but in this, as in many other matters of life, it is true that "fools rush in where angels fear to tread." Perhaps the too hasty rush at the experimental verification of Foucault's law may account for some persons in England, whose opinions when given with due deliberation are entitled to respect, having allowed themselves to express doubts (which I understand, however, have been since retracted) as to the truth of the law itself. In Paris there was no difference of opinion among such men as Lamé, Poinsol, Binet, Leonville, Sturm, Chasles, Bruvues, I believe Arago, Hermite, and many others with whom I conversed on the subject, except as to the best mode of making the theory popularly intelligible."