To clearly appreciate the following popular explanation, it will be necessary for the reader to convince himself of one property of the pendulum, viz., that of constantly vibrating in the same plane. Let it be imagined that a pendulum is suspended over a common table, _the parts bearing the pendulum being also attached to the table_. Suppose, also, that the table can move freely on its centre like a music-stool: the pendulum being put in motion will continue to move in the same plane between the eye and any object on the walls of the room, although the table is made to revolve, and during one revolution will have _radiated_ through the whole circumference. A few moments' reflection are only necessary to prove this. [Illustration: FIGURE 1. Rotation of the earth--Diagram 1] The above figure represents a plane or table on the top of a globe, or at the north pole of the earth. To this table are fixed two rods, from which is suspended a pendulum, moving freely in any direction. The pendulum is made to vibrate in the path _a b_; it will continue to vibrate in this line, and have no apparent circular or angular motion until the globe revolves, when it will appear to have vibrated through the entire circle, _to an object fixed on the table and moving with it_. It is scarcely necessary to say the circular motion of the pendulum is only apparent, since it is the table that revolves--the apparent motion of the pendulum in a circle being the same as the apparent motion of the land to a person on board ship, or the recession of the earth to a person in a balloon. The pendulum vibrates always in the same plane at the pole, and in planes parallel to each other at any intermediate point. [Illustration: FIG. 2. Rotation of the earth--Diagram 2] Fig. 2 represents the earth or a globe revolving once in twenty-four hours on its axis (S N). It is divided, on its upper half, by lines parallel to each other, representing the latitudes 60 degrees, 30 degrees, and the equator, where the latitude is nothing. The lines _a b_, at 90, 60, 30, and 0 represent the planes of those latitudes; or, in more familiar terms, tables, over which a pendulum is supposed to vibrate, and moving with them in their revolutions round the axis (S N). This being clearly understood, the next object is to show how the pendulum moves round the tables, for each of the latitudes; also to show the gradual diminution of its circular motion as it approaches the equator (E E), where, as was before observed, the latitude is nothing.