Your correspondent appears to consider that the only fact asserted by the propounders of the theory, is a variation in the plane of oscillation, caused by "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;" the probable existence of which he proves by imagining a pendulum suspended over a point half-way between London and Edinburgh, and set in motion by being drawn towards and retained over London, and thence dismissed on its course. It is clear that in such a case the pendulum would at starting be impressed with the same velocity of motion in an eastern direction which the retaining power in London had, and that its path would be the result of this force compounded with that given by gravity in its line of suspension, _i.e._ towards the north, and its course would therefore be one subject to easy calculation. I should imagine that this disturbing force arising from the excess of eastern velocity possessed by the starting point over that of suspension, would be inappreciable after a few oscillations; but at all events it is evident that it might readily be avoided by setting the pendulum in motion by an impulse given beneath the point of suspension, by giving to it a direction east and west as suggested by A. E. B., or by several other expedients which must occur to a mathematician. Your correspondent proceeds by requiring that there should be shown "reasonable ground to induce the belief that the ball is really free from the attraction of each successive point of the earth's surface," and is not as "effectually a partaker in the rotation of any given point" as if it were fixed there; or that "the duration of residence" necessary to cause such effect should be stated. Now I certainly am aware of no force by which a body unconnected with the earth would have any tendency to rotate with it; gravity can only act in a direct line from the body affected to the centre of the attracting body, and the motion in the direction of the earth's rotation can only be gained by contact or connexion, however momentary, with it. The onus of proving the existence of such a force as A. E. B. alludes to, must surely rest with him, not that of disproving it with me. What the propounders of this theory claim to show is, I humbly conceive, this,--that the direction in which a pendulum oscillates is _constant_, and not affected by the rotation of the earth beneath it: that as when suspended above the pole (where the point of suspension would remain fixed) the plane of each oscillation would make a _different_ angle with any given meridian of longitude, returning to its original angle when the diurnal rotation of the earth was completed; and as when suspended above the equator, where the point of suspension would be moved in a right line, or, to define more accurately, where the plane made by the motion of a line