PENDULUM DEMONSTRATION OF THE EARTH'S ROTATION.
(Vol. iv., pp. 129. 177.)
Your correspondent A. E. B. appears, by his suggestion regarding Foucault's theory, to have rendered confusion worse confounded, mystery more mysterious. He says:
"If the propounders of this theory had from the first explained, that they do not claim for the plane of oscillation an exemption from the general rotation of the earth, but only the difference of rotation due to the excess of velocity with which one extremity of the line of oscillation may be affected more than the other, it would have saved a world of fruitless conjecture and misunderstanding."
This supposition makes an effect, which it is difficult to believe in, into one utterly impossible to conceive. It is hard enough to credit the theory, that the plane of oscillation of a pendulum is partially independent of the rotatory motion of the earth, but still not impossible, considering that the effect of the presumed cause is not inconsistent with the results of à priori calculation. For instance, during the swing of a two-seconds pendulum, the angular motion of the earth will have been 1', or thereabouts, which, supposing the oscillation to be independent, would produce an appreciable angle on an index circle placed concentric with the pendulum, and at right angles to its plane of oscillation.
But as to A. E. B.'s theory, which supposes the variation of the pendulum's plane to be "due to the excess of velocity with which one extremity of the line of oscillation may be affected more than the other," it appears to me quite untenable for a moment. Let him reduce it to paper, and find what difference of velocity there is on the earth's surface at the two ends of a line of ten feet, the assumed length of the arc of a two-seconds pendulum,—a larger one, I presume, than that used by Foucault in his cellar,—and I believe he will find it to be practically nothing.
I confess I have had no faith in this theory from the first; the effect, if any and constant, I believe to be magnetic. The results of experiments have been stated from the first very loosely, and the theory itself has been put forth very indistinctly, and not supported by any name of eminence, except that of Professor Powell.
In the meantime, and until some competent authority has pronounced on the point, I propose that such of your readers as are interested in the question make experiments for themselves, dividing them into four classes, viz., with the plane of oscillation E. and W., N. and S., N.E. and S.W., N.W. and S.E.; take the mean of a great many, and communicate them to the editor of "NOTES AND QUERIES;" and I venture to say that such a collection will do more towards confirming or disproving the theory absolutely, than all the papers we have yet seen on the subject.
I am myself about to make experiments with a twenty-five feet pendulum.
H. C. K.
—— Rectory, Hereford, Sept. 8. 1851.