Of Motive Forces

THIRD DEFINITION.

Pliny Book 2. Chapter 103.

All parts of this element are directly contrary to the nature of fire; moisture is in water as heat is in fire, and from these two contrary parts, admirable machines can be made, as some will be shown hereafter. Water is also said to be heavy, yet not all waters are of the same weight: Pliny Pliny the Elder, a Roman author and natural philosopher. recounting the nature of several different waters, says that in the territory of Carra in Spain, there are two springs near each other whose natures are so diverse that everything placed on one sinks to the bottom, and the other carries everything put upon it, as do the Lake of Sodom The Dead Sea, famous for its high salt content which allows people to float easily. and the River Arethusa; the reason for this comes from the weight of the water, which, being heavier in comparison than the body placed within it, casts it upward, because the greater weight seeks to occupy the lowest place, as we see for example that iron and lead float atop quicksilver mercury, for quicksilver, being the heavier in an equal volume, seeks to hold the lowest place; and conversely, the lightest water cannot support any heavy thing. Pliny further recounts as a marvelous thing that a pumice stone floats on water when it is in a large piece, but when turned to powder it sinks to the bottom; the cause of this is easy to give, for the pumice stone, being of a porous nature and filled with air, cannot sink because the air enclosed within the said pores seeks to hold the high place, as is its nature; but when the said stone is in powder, and there is no longer air mixed with it to carry it, it will go to the bottom. The same is true of several other things; pieces of wood also float on water, which when sawn into powder sink to the bottom as soon as the sawdust or powder is soaked with water. By similar reasoning, large ships loaded with lead and other heavy merchandise are supported on the water because the air within the concavity the hollow hull of the said ships prevents them from sinking. As for what is said about it being a flowing element, this is well enough known by experience; it remains to show how it cannot be compressed as fire and air can, and I will give an example of this. Let there be a round copper vessel containing three or four measures of water, in which there is a small hole to fill the said vessel; and afterward, if one fits the end of a syringe to the hole of the said vessel and attempts to push the water from the said syringe into the vessel, it will be found in effect that it will not be possible to make any more water enter than what entered voluntarily. Water, therefore, cannot be compressed to create any violence In 17th-century physics, "violence" refers to forceful, rapid, or unnatural movement/pressure. as air or fire does; but the violence of water consists in its weight when it descends from high places, or else when it is moved by the wind. One cannot say with certainty whether water is hot, participating in the nature of fire, or if it is cold, participating in the nature of air; my opinion is that it is neither hot nor cold by its nature, but being aided by fire or air, it receives heat or cold by accident.