I trust you will be able to obtain the use of a globe,[1] to aid you in the study of the foregoing definitions, or doctrine of the sphere; but if not, I would recommend the following easy device. To represent the earth, select a large _apple_, (a melon, when in season, will be found still better.) The eye and the stem of the apple will indicate the position of the two poles of the earth. Applying the thumb and finger of the left hand to the poles, and holding the apple so that the poles may be in a north and south line, turn this globe from west to east, and its motion will correspond to the diurnal movement of the earth. Pass a wire or a knitting needle through the poles, and it will represent the _axis_ of the sphere. A circle cut around the apple, half way between the poles, will be the _equator_; and several other circles cut between the equator and the poles, parallel to the equator, will represent _parallels of latitude_; of which, two, drawn twenty-three and a half degrees from the equator, will be the _tropics_, and two others, at the same distance from the poles, will be the _polar circles_. A great circle cut through the poles, in a north and south direction, will form the _meridian_, and several other great circles drawn through the poles, and of course perpendicularly to the equator, will be secondaries to the equator, constituting meridians, or _hour circles_. A great circle cut through the centre of the earth, from one tropic to the other, would represent the _plane_ of the ecliptic; and consequently a line cut round the apple where such a section meets the surface, will be the terrestrial _ecliptic_. The points where this circle meets the tropics indicate the position of the _solstices_; and its intersection with the equator, that of the _equinoctial points_. The _horizon_ is best represented by a circular piece of pasteboard, cut so as to fit closely to the apple, being movable upon it. When this horizon is passed through the poles, it becomes the horizon of the equator; when it is so placed as to coincide with the earth's equator, it becomes the horizon of the poles; and in every other situation it represents the horizon of a place on the globe ninety degrees every way from it. Suppose we are in latitude forty degrees; then let us place our movable paper parallel to our own horizon, and elevate the pole forty degrees above it, as near as we can judge by the eye. If we cut a circle around the apple, passing through its highest part, and through the east and west points, it will represent the _prime vertical_.