A concise collection of experimental and theoretical essays that examine the behavior of fluids and the effects of atmospheric pressure, presenting precise experiments—such as barometric observations and tests on vacuums—and arguments about hydrostatic equilibrium, the transmission of pressure, and the existence of a vacuum; combining careful measurement, thought experiments, and mathematical reasoning, these treatises show how pressure varies with depth, how air exerts measurable force, and how instruments like the mercury barometer can probe atmospheric phenomena, helping to establish principles foundational to experimental physics.