Chap. i. PHILOSOPHY.

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more continually towards the earth, whither it is directedby its weight: but since, when the weight of a body doesnot alter the direction of its motion, it always moves ina straight line, without doubt in this ether cafe the body’sdeclining from its first course is no more, than what is cau-sed by its weight alone. As this appears at first sight to beunquestionable, so we shall have a very distinct proof thereofin the next chapter, where the oblique motion of bodies willbe particularly considered.

II. Thus we fee how the first of the laws of motionagrees with what appears to us in moving bodies. Buthere occurs this farther consideration, that the real and ab-solute motion of any body is not visible to us : for weare our selves also in constant motion along with theearth whereon we dwell; insomuch that we perceive bo-dies to move so far only, as their motion is different fromour own. When a body appears to us to lie at rest, inreality it only continues the motion, it has received, withoutputting forth any power to change that motion. If wethrow a body in the course or direction, wherein we arecarried our selves; so much motion as we seem to havegiven to the body, so much we have truly added to themotion, it had, while it appeared to us to be at rest. Butif we impel a body the contrary way, although the bodyappears to us to have received by such an impulse as muchmotion, as when impelled the qther way; yet in this cafe wehave taken from the body so much real motion, as we seemto have given it. Thus the motion, which we fee in bodies,

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