The laws of nature are the rules according to which the effects are produced; but there must be a cause which operates according to these rules. The laws of navigation never navigated a ship. The rules of architecture never built a house.

The science of government is my duty. ... I must study politics and war that my sons may have liberty to study mathematics and philosophy. My sons ought to study mathematics and philosophy, geography, natural history, naval architecture, navigation, commerce, and agriculture, in order to give their children a right to study painting, poetry, music, architecture, statuary, tapestry, and porcelain.

There is more evidence to prove that saltiness [of the sea] is due to the admixture of some substance ... It is this stuff which makes salt water heavy (it weighs more than fresh water) and thick. The difference in consistency is such that ships with the same cargo very nearly sink in a river when they are quite fit to navigate in the sea. This circumstance has before now caused loss to shippers freighting their ships in a river. That the thicker consistency is due to an admixture of something is proved by the fact that if you make strong brine by the admixture of salt, eggs, even when they are full, float in it. It almost becomes like mud; such a quantity of earthy matter is there in the sea.
[Aristotle recognised the different density of fresh (river) or salty (sea) water. He describes an experiment using an egg (which sinks in fresh water) that floats in a strong brine solution.]

To find fault with our ancestors for not having annual parliaments, universal suffrage, and vote by ballot, would be like quarrelling with the Greeks and Romans for not using steam navigation, when we know it is so safe and expeditious; which would be, in short, simply finding fault with the third century before Christ for not being the eighteenth century after. It was necessary that many other things should be thought and done, before, according to the laws of human affairs, it was possible that steam navigation should be thought of. Human nature must proceed step by step, in politics as well as in physics.

[The steamboat] will answer for sea voyages as well as for inland navigation, in particular for packets, where there may be a great number of passengers. He is also of opinion, that fuel for a short voyage would not exceed the weight of water for a long one, and it would produce a constant supply of fresh water. ... [T]he boat would make head against the most violent tempests, and thereby escape the danger of a lee shore; and that the same force may be applied to a pump to free a leaky ship of her water. ... [T]he good effects of the machine, is the almost omnipotent force by which it is actuated, and the very simple, easy, and natural way by which the screws or paddles are turned to answer the purpose of oars.
[This letter was written in 1785, before the first steamboat carried a man (Fitch) on 27 Aug 1787.]