SEC. III.] OF NATURAL PHILOSOPHY. 123 this ratio, and (by Cor. 6 of this Prop, and Prop. IV) the same proportion holds in all distances. And hence, also, in a parabola, the velocity is everywhere equal to the velocity of a body revolving in a circle at half the distance ; in the ellipsis it is less, and in the hyperbola greater. COR. S. The velocity of a body revolving in any conic section is to the velocity of a body revolving in a circle, at the distance of half the princi pal latus rectum of the section, as that distance to the perpendicular let fall from the focus on the tangent of the section. This appears from Cor. 5. COR. 9. Wherefore since (by Cor. 6, Prop. IV), the velocity of a body revolving in this circle is to the velocity of another body revolving in any other circle reciprocally in the subduplicate ratio of the distances; there fore, ex czqiiO) the velocity of a body revolving *in a conic section will be to the velocity of a body revolving* in a circle at the same distance as a mean proportional between that common distance, and half the principal latus rectum of the section, to the perpendicular let fall from the common focus upon the tangent of the section. PROPOSITION XVII. PROBLEM IX. Supposing the centripetal force to be reciprocally proportional to the squares of the distances of places from the centre, and that the abso lute quantity of that force is known ; it is required to determine t/te line which a body will describe that is let go from a given place with a given velocity in the direction of a given right line. Let the centripetal force tending to the point S be such as will make the body p revolve in any given orbit pq ; and suppose the velocity of this body in the place p is known. Then from the place P suppose the body P to be let go with a given ve locity in the direction of the line PR ; but by virtue of a centripetal force to be immediately turned aside from that right line into the conic section PQ,. This, the right line PR will therefore touch in P. Suppose likewise that the right line pr touches the orbit pq in p ; and if from S you suppose perpendiculars let fall on those tangents, the principal latus rectum of the conic section (by Cor. 1, Prop. XVI) will be to the principal latus rectum of that orbit in a ratio compounded of the duplicate ratio of the perpendiculars, and the duplicate ratio of the velocities ; arid is therefore given. Let this latus rectum be L ; the focus S of the conic