In many cases the experiments have not shown a change at all in the plane of oscillation of the pendulum; in others the alteration in the plane of vibration has been in the _wrong direction_; and very often the _rate of variation_ has been altogether different to that which theory indicated. The following is a case in illustration:--“On Wednesday evening the Rev. H. H. Jones, F.R.A.S., exhibited the apparatus of Foucault to illustrate the diurnal rotation of the Earth, in the Library Hall of the Manchester Athenæum. The preparations were simple. A circle of chalk was drawn in the centre of the floor, immediately under the arched skylight. The circle was exactly 360 inches in its circumference, every inch being intended to represent one degree. According to a calculation Mr. Jones had made, and which he produced at the Philosophical Society six weeks ago, the plane of oscillation of the pendulum would, at Manchester, diverge about one degree in five minutes, or perhaps a very little less. He therefore drew this circle exactly 360 inches round, and marked the inches on its circumference. The pendulum was hung from the skylight immediately over the centre of the circle, the point of suspension being 25 feet high. At that length of wire, it should require 2¹⁄₂ seconds to make each oscillation across the circle. The brazen ball, which at the end of a fine wire constituted the pendulum, was furnished with a point, to enable the spectator to observe the more easily its course. A long line was drawn through the diameter of the circle, due north and south, and the pendulum started so as to swing exactly along this line; to the westward of which, at intervals of three inches at the circumference, two other lines were drawn, passing through the centre. According to the theory, the pendulum should diverge from its original line towards the west, at the rate of one inch or degree in five minutes. This, however, Mr. Jones explained, was a perfection of accuracy only attainable in a vacuum, and rarely could be approached where the pendulum had to pass through an atmosphere subject to disturbances; besides, it was difficult to avoid giving it some slight lateral bias at starting. In order to obviate this as much as possible, the steel wire was as fine as would bear the weight, ¹⁄₃₀th of an inch thick; and the point of suspension was adjusted with delicate nicety. An iron bolt was screwed into the frame-work of the skylight; into it a brass nut was inserted--the wire passed through the nut (the hollow sides of which were bell-shaped, in order to give it fair play), and at the top the wire ended in a globular piece, there being also a fine screw to keep it from slipping. * * * The pendulum was gently drawn up to one side, at the southern end of the diametrical line, and attached by a thread to something near. When it hung quite still the thread was burnt asunder, and the pendulum began to oscillate to and fro across the circle.