LXX. It is very probable, and perhaps more than probable, that the ascent of light bodies, is caused by the descent of heavy ones; because the heavy body has power, in consequence of the impetus of its descent, to occupy the lower station, where meeting with the light body, it obliges him to leave that situation, and to mount upwards; for many argue with great colour of reason, that there is no such thing as absolute lightness to be conceived in any body whatever, nor is such a quality necessary, a respective or comparative lightness, being sufficient to answer every purpose. Thus we say a body is light, not because it is void of gravity, but because it is less heavy than the one with which we compare it. In the same manner we say the air is light, not because it is not ponderous, but because it is less so, than earth, water, and all the other bodies that surround us; and that no other levity but the respective, is necessary to cause bodies which we call light to ascend, may be clearly seen in the case of oil, which notwithstanding that it is heavy, if you pour a quantity of water into the vessel where it is, the water on account of its superior gravity, will occupy the inferior station, and oblige the oil which was at the bottom to ascend. The same thing happens with regard to air. If you dig a ditch to any depth in dry ground, the air will descend and occupy the whole of it; and there is no other way to dispossess the air from the bottom, and to make it rise upwards, but conveying water into the ditch, or by throwing into it some body that is heavier than the air. LXXI. It is not owing to the principles of physics, but to experience, that we understand the little we know of this matter; and with respect to this little, there still remains great difficulties for philosophers to contemplate; and the greatest of all, is ascertaining the cause of the ascent of vapours to the region of the air. It is certain, that vapours are nothing else but water, resolved into exceedingly minute particles. The water however being heavier than air, how can the water arise to the height occupied by the clouds? Every particle of water notwithstanding it weighs very little, is much heavier than a particle of air of equal size, and the greater or less gravity of liquids, for the purpose of their impelling one another, is computed in the gross, and not according to the proportion of globules of equal size; and we know that a pound of water, will cause a quarter of an hundred of oil to rise in a vessel.