Chap. L. PHILOSOPHICAL DISCOVERIES. i 45

able to the course of things, and to perpetual experience, thatthe third law of motion be extended generally to all fortsof powers that take place in nature, those of attraElion andrepuljion as well as others, (and not to be a supposition arbi-trarily introduced by Sir Isaac Newton ;) when those powers arefound to depend upon the bodies that are said to attraEl or re-pels as well as upon those that are attraSled or repelled. Wefind the loadstone attracts iron, and that iron attracts the load-stone with equal force ; and because they attract each otherequally, they remain at rest when they come into contact. If amountain by its gravity pressed upon the earth, and the earthdid not react equally on the mountain ; then the mountainwould neceflarily carry the earth before it, by its presiure,with a motion accelerated in infinitum. The fame is to be saidof a stone, or the least part of the earth, as well as of a moun-tain. Bodies act upon light in proportion to their density,cœterts partbus, by refracting it when it enters into them ; andconversely, light acts upon bodies by heating them and puttingtheir parts in motion. This equality of aEUon and reaElion ob-tains so generally, that when any new motion is produced byany power or agent in nature, there is always a correspondingequal and opposite motion produced by its reaElion at the fametime, or some equal motion in the fame direction destroyed.When from an engine a weight is thrown, the engine reactswith an equal force on the earth or air. If it was not for thislaw, the state of the centre of gravity of the earth would beaffected by every action or impulse of every power or agentupon it. But by virtue of this law, the state of the centre ofgravity of the earth, and the general course of things, is pre-served, independent of any motions that can be produced at ornear its surface, or within its bowels. By the fame law, thestate of the leffer systems of the planets, and the repose os the•general system, is preserved, without any disturbance from the

U actions