Page:This New Ocean, a history of Project Mercury, Swenson, Grimwood, Alexander (NASA SP-4201).djvu/19



The action of centripetal forces as advanced by Isaac Newton: "That by means of centripetal forces the planets may be retained in certain orbits, we may easily understand, if we consider the motions of projectiles; for a stone that is projected is by the pressure of its own weight forced out of the rectilinear path, which by the initial projection alone it should have pursued, and made to describe a curved line in the air; and through that crooked way is at last brought down to the ground; and the greater the velocity is with which it is projected, the farther it goes before it falls to the earth. We may therefore suppose the velocity to be so increased, that it would describe an arc of 1,2, 5, 10, 100, 1,000 miles before it arrived at earth, till at last, exceeding the limits of the earth, it should pass into space without touching it." 4