about a permanent axis, no force intervene to alter the original motion, the axis will always preserve the same direction in space, as well as the same position in the body, a circumstance alluded to in the lectures, (page 78.)
Suppose, then, a body to be set rotating about a permanent axis and to be mounted so that the force of gravity does not interfere with the rotation; suppose, also, that at the commencement of the motion the axis points to some star; then, if it be true that the star does not move, the axis will always point to it so long as the rotation lasts. If it be true that the apparent diurnal motions of the stars are due to an actual motion of the earth; then to an observer on the earth's surface, the axis will appear to move so as to follow the star. But if, on the other hand, the star move while the earth remains at rest, no such apparent change in the position of the axis will be observed. Thus we have the means of testing, by direct experiment, the truth of the conclusion arrived at in lecture II, (page 78,) that it is the earth which revolves and not the stars.
The following is the description of an instrument, contrived by M. Foucault, for the purpose of making such an experiment as we have just mentioned. DD', Figure 69, is a heavy metallic disc, mounted on an axis which passes through O, the centre of the disc, and is perpendicular to its two sides. The extremities of this axis terminate in pivots CC', which fit into holes made at opposite extremities of the diameter of a circular ring BCB'C', which is furnished with two knife edges (similar to those of a balance) at B and B', and so arranged that BB' is the diameter of the ring perpendicular to CC'. The knife edges rest in holes made at opposite extremities
