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22 diameter, than grains of sand on a globe of a foot diameter. The Earth, however, independently of this irregularity of surface, is not a perfect sphere; the equatorial diameter being greater than the polar by 34 miles. This is caused by the tendency of the matter composing the globe to fly out at the point most distant from its axis, owing to the velocity of its diurnal revolution. If the Earth’s axis (or line joining the poles) were perpendicular to the ecliptic (or plane of its orbit), the Sun would shine directly upon the equator in all parts of the Earth’s revolution round it; and there would consequently be no change of seasons, or of the length of the days and nights throughout the year. But this is not the case, for the axis of the Earth is inclined 285° from the perpendicular, and is always in the same direction; so that, the poles being alternately directed towards the Sun, he shines in succession opposite all the parts between the tropics of Cancer and Capricon. DIAGRAM XYII. Represents the Earth in four dilferent positions. On the 21st of June, the north pole inclines towards the Sun, and it is summer in the northern hemisphere; the \ Sun then shines vertically upon the tropic of Cancer : whilst on the 21st of December, it is winter in the northern hemisphere, and summer in the southern, the south pole being then turned towards the Sun, and the Sun shining vertically upon the tropic of Capricorn. On March 21st and September 21st, both poles are equally turned towards the Sun, which then shines ver- tically on the equator, so that each hemisphere receives an equal portion of light and heat, and the days and nights are equal all over the globe. The latter periods are called the vernal and autumnal equinoxes ; June 21st, is called the summer solstice, and Dec. 21st the winter solstice. Now, it will be seen that from March 21st to Sept. 21st, the greatest part of the upper hemi- sphere is enlightened by the Sun, so that the days will