286 THE MATHEMATICAL PRINCIPLES [BOOK II BD X V2 one to another as the area r-^ — , and the difference of the areas DET AD and A6NK ; and moreover since the space, in a non-resisting medium, is BD X V2 perpetually as Tu~~> an(^ tne sPace> in a resisting medium, is perpetu ally as the difference of the areas DET and A&NK ; it necessarily follows, that the spaces, in both mediums, described in any equal times, are one to BD X V2 another as that area 7-5 — » an(^ ^he difference of the areas DET and A6NK. Q.E.D. SCHOLIUM. The resistance of spherical bodies in fluids arises partly from the tena city, partly from the attrition, and partly from the density of the medium. And that part of the resistance which arises from the density of the fluid is, as I said, in a duplicate ratio of the velocity ; the other part, which arises from the tenacity of the fluid, is uniform, or as the moment of the time ; and, therefore, we might now proceed to the motion of bodies, whicli are resisted partly by an uniform force, or in the ratio of the moments of the time, and partly in the duplicate ratio of the velocity. But it is suf ficient to have cleared the way to this speculation in Prop. VIII and IX foregoing, and their Corollaries. For in those Propositions, instead of the uniform resistance made to an ascending body arising from its gravity, one may substitute the uniform resistance which arises from the tenacity of the medium, when the body moves by its vis insita alone ; and when the body ascends in a right line, add this uniform resistance to the force of gravity, and subduct it when the body descends in a right line. One might also go on to the motion of bodies which are resisted in part uni formly, in part in the ratio of the velocity, and in part in the duplicate ratio of the same velocity. And I have opened a way to this in Prop. XIII and XIV foregoing, in which the uniform resistance arising from the tenacity of the medium may be substituted for the force of gravity, or be compounded with it as before. But I hasten to other things.