_Mrs. B._ The only alteration required is to lengthen the pendulum in one case, and to shorten it in the other; for the velocity of the vibrations of a pendulum depends on its length; and when it is said that a pendulum vibrates quicker at the pole than at the equator, it is supposed to be of the same length. A pendulum which vibrates seconds in this latitude is about 39-1/7 inches long. In order to vibrate at the equator in the same space of time, it must be somewhat shorter; and at the poles, it must be proportionally lengthened. The vibrations of a pendulum, resemble the descent of a body on an inclined plane, and are produced by the same cause; now you must recollect, that the greater the perpendicular height of such a plane, in proportion to its length, the more rapid will be the descent of the body; a short pendulum ascends to a greater height than a larger one, in vibrating a given distance, and of course its descent must be more rapid. I shall now, I think, be able to explain to you the cause of the variation of the seasons, and the difference in the length of the days and nights in those seasons; both effects resulting from the same cause. In moving round the sun, the axis of the earth is not perpendicular to the plane of its orbit. Supposing this round table to represent the plane of the earth's orbit, and this little globe, the earth; through this I have passed a wire, representing its axis and poles. In moving round the table, I do not hold the wire perpendicular to it, but obliquely. _Emily._ Yes, I understand, the earth does not go round the sun in an upright position, its axis is slanting or oblique; and, it of course, forms an angle with a line drawn perpendicular to the plane of the earth's orbit. _Mrs. B._ All the lines, which you learnt in your last lesson, are delineated on this little globe; you must consider the ecliptic as representing the plane of the earth's orbit; and the equator, which crosses the ecliptic in two places, then shows the degree of obliquity of the axis of the earth; which amounts to 23-1/2 degrees, very nearly. The points in which the ecliptic intersects the equator, are called the equinoctial points. But I believe I shall render the effects of the obliquity of the earth's axis clearer to you, by the revolution of the little globe round a candle, which shall represent the sun. (Plate IX. fig. 2.) As I now hold it, at A, you see it in the situation in which it is in the midst of summer, or what is called the summer solstice, which is on the 21st of June. _Emily._ You hold the wire awry, I suppose, in order to show that the axis of the earth is not upright?