Although the manner of the Shape of several Creatures, is the chief cause of their _Ascent_, and _Descent_; yet, Gravity and Levity, doth occasion more or less Agility: for, a Heavy Body shall descend with more force, than a Leight Body ascend: and the reason is, not only that there may be more Parts in a Heavy Body, than a Leight; but, that in a Descent, every Corporeal Motion seems to press upon each other; which doubles and trebles the Strength, Weight, and Force, as we may perceive in the Ascending and Descending of the Flight of Birds, especially of Hawks; of which, the weight of the Body is some hindrance to the Ascent, but an advantage to the Descent: but yet, the Shape of the Bird hath some advantage by the Weight, in such sort, that the Weight doth not so much hinder the Ascent, as it doth assist the Descent. CHAP. VIII. Of several sorts of Densities and Rarities, Gravities and Levities. There are different sorts of Densities and Rarities, Softness and Hardness, Levities and Gravities: as for example, The density of Earth is not like the density of Stone; nor the density of Stone, like the density of Metal: nor are all the Parts of the Earth dense alike; nor all Stones, nor all Metals; as we may perceive in Clay, Sand, Chalk, and Lime-Grounds. Also, we may perceive difference between Lead, Tynne, Copper, Iron, Silver, and Gold; and between Marble, Alablaster, Walling-Stone, Diamonds, Crystals, and the like: and so much difference there is between one and the same kind, that some particulars of one sort, shall more resemble another kind, than their own: as for example, Gold and Diamonds resemble each other's Nature, more than Lead doth Gold; or Diamonds, Crystal; I say, in their Densities. Also, there is a great difference of the Rarity, Gravity, and Levity of several sorts of Waters, and of several sorts of Air. CHAP. IX. Of VEGETABLES.