The double motion of the Planets,—progressive and rotary,—is perhaps one of the most difficult things for young pupils properly to understand, without some popular illustration. If the teacher, therefore, has no orrery to show this motion to his pupils, he may compare it to a screw which is turned round whilst it suffers at the same time a progressive motion; or perhaps with more propriety to a spinning-top, which is continually turning on its Axis, while at the same time it describes large circles.[4] § 21. If you have well understood what has just been said, you will be able to comprehend, that _our Earth, while it is performing its great journey round the Sun in Three Hundred and Sixtyfive days, is, at the same time, every Twentyfour hours turning on its Axis_. This _rotary_ motion of our Earth on its Axis is the cause of the successive changes of day and night; that portion of the Earth which is turned toward the Sun having always day, when the other, which is turned away from him, has night. This is again a wise dispensation of God’s providence. For if the Earth would always keep the same relative position to the Sun, then that portion of it, which would then be turned toward the Sun, would have continual day, whilst the other, which would then be turned away from him, would be enveloped in perpetual darkness. But as it is now arranged by the Earth’s rotation on its Axis, most every portion of its surface must at least _once_ every Twentyfour hours be turned toward the Sun and receive from him light and heat. Without this, one great half of our Earth would have a perpetual winter, destructive to plants and animals, while an everlasting summer would scorch the other half and render it equally unfit for the support of man. The following Diagram, Plate IX, may serve to give you an idea of the Earth’s rotation round its Axis, and the alternate succession of Day and Night, resulting from it. When the Earth is situated as represented in the Diagram, then that portion of it, which is marked A, will have Day, because it is turned toward the Sun; and the portion marked B, will have Night. But in the course of the next Twelve hours the order will be reversed. The portion which is marked B, will be turned toward the Sun and have day, whilst the portion A will be turned from him, and have night.