The source of mechanical power is the sun which exhales vapors that descend in rain, to turn mills, or which causes winds to blow by the unequal rarefaction of the atmosphere.

The velocity with which a denser fluid flows into a rarer one is equal to the velocity a heavy body acquires in falling through a height equal to the difference of altitude of two columns of the heavier fluid of such heights as will produce the respective pressures; and, therefore, when the difference of pressure or amount of rarefaction in the chimney is known, it is easy to tell the velocity of motion which ought to be produced by it.

Nothing renders men modest but a just knowledge how to compare themselves with others; and where that is wanting impudence supplies the place of it, for there is no vacuum in the minds of men, and commonly, like other things in Nature, they swell more with rarefaction than condensation.