JLET. XV.

OF THE EARTH.

255

finds itself stretched by a considerable force,which force is exerted upon it by the stone, inits endeavours to recede from the center; andif the stone be disengaged from the sting, by asudden stop of the hand, it will immediatelymanifest the tendency which it has to leave thisconstrained circular orbit, by its proceeding di-rectly forwards in a straight line.

Besides this, there is another force, which iscalled centripetal; and is so denominated, becauseit is directed towards the center, and acts indirect opposition to the former. This force, inthe present case, is the same as gravity; thenature of which may be thus explained. Allheavy bodies, when left to themselves, are ob-served to fall towards the earth in straight lines,which are perpendicular to the horizon; andif those lines were continued, it is plain, fromthe nature of a globe, that they would all passthrough the earth’s center. Every part of theearth, therefore, gravitates towards the center;and as this force is found to be about two hun-dred and eighty-nine times greater than theopposite one, or that which arises from the ro-tation of the earth upon its axis, a certain ba-lance will constantly be maintained betweenthem, and the earth will assume such a figure,as would naturally result from the difference ofthese two contrary and opposite forces.

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