244 Sir ISAAC NEWTON’s Book III.

jected in. the air, we do not fee it fall in the perpendicular to-wards the earth, but we fee it falling every moment from thetangent to the curve, that is, from the direction in which itwould have moved if its gravity had not acted for that mo-ment. And this proof we have of the moon’s gravity : fortho* we do not fee her falling directly towards the earth in aright line, yet we observe her descending every moment towardsthe earth from the right line which was the direction of hermotion at the beginning of that moment; and this is no lessevidently a proof of her being acted upon by gravity, or somepower like to it, than her rectilineal descent would be wasme allowed to fall freely towards the earth.

. 3. If we had engines of a sufficient force, bodies mightbe projected from them so as not only to be carried a vast waywithout falling to the earth, but so as to move over a quarterof a great circle of it, or (abstracting from the effects of the air’sresistance) so as to move round the whole earth without touch-ing it, and, after returning to their first place, commence anew revolution with the fame force they first received from theengine, and after that a third, and thus revolve as a moon orsatellite round the earth for ever. If this could be effectednear the earth’s surface it might be done higher in the air oreven as high as the moon, could the engine, or an equivalentpower, be carried up and made to act there. By increasingthe force of the power, a body proportionally larger might bethus projected; and, by a power sufficiently great, a heavy bodynot inferior to the moon might be put in motion at first;which, being perpetually restrained by it gravity from going offin a right line, might revolve for ever about the earth. ThusSir Isaac Newton saw that the curvilineal motion of the moonin her orbit, and of any projectile at the surface of the earth,

were-