What is the SI unit of pressure?

The pascal, one newton per square metre - which is a very small amount of pressure. Atmospheric pressure at sea level is about 101,325 pascals, which is why weather reports use hectopascals and engineers use megapascals, and why almost nobody uses the base unit unadorned.

Blaise Pascal, in the 1640s, established that air has weight and a top by having a barometer carried up a mountain and showing the mercury column fell. He also worked out the principle that pressure applied to a confined fluid is transmitted undiminished in all directions - which is the entire basis of hydraulics, and why a small force on a small piston becomes a large force on a large one. Every car brake and every excavator arm runs on it.

He then gave up mathematics. After a religious experience in 1654 he devoted himself to theology and wrote the Pensees, returning to mathematics only briefly and reportedly to distract himself from toothache - during which he worked out significant results on the cycloid. He also invented one of the first mechanical calculators, at nineteen, to help his father with tax accounts, and died at 39.