Let us retain the earth a moment at the point D, and consider how it will be affected by the combined action of the two forces in its new situation. It still retains its tendency to fly off in a straight line; but a straight line would now carry it away to F, whilst the sun would attract it in the direction D S; how then will it proceed? _Emily._ It will go on in a curve-line, in a direction between that of the two forces. _Mrs. B._ In order to know exactly what course the earth will follow, draw another parallelogram similar to the first, in which the line D F describes the force of projection, and the line D S that of attraction; and you will find that the earth will proceed in the curve-line D G. _Caroline._ You must now allow me to draw a parallelogram, Mrs. B. Let me consider in what direction will the force of projection now impel the earth. _Mrs. B._ First draw a line from the earth to the sun representing the force of attraction; then describe the force of projection at a right angle to it. _Caroline._ The earth will then move in the curve G I, of the parallelogram G H I K. _Mrs. B._ You recollect that a body acted upon by two forces, moves through a diagonal, in the same time that it would have moved through one of the sides of the parallelogram, were it acted upon by one force only. The earth has passed through the diagonals of these three parallelograms, in the space of three months, and has performed one quarter of a circle; and on the same principle it will go on till it has completed the whole of the circle. It will then recommence a course, which it has pursued ever since it first issued from the hand of its Creator, and which there is every reason to suppose it will continue to follow, as long as it remains in existence. _Emily._ What a grand and beautiful effect resulting from so simple a cause! _Caroline._ It affords an example, on a magnificent scale, of the curvilinear motion, which you taught us in mechanics. The attraction of the sun is the centripetal force, which confines the earth to a centre; and the impulse of projection, the centrifugal force, which impels the earth to quit the sun, and fly off in a tangent.