Altimeter and tunnelling shield: what they have in common

1. Step 1. Altimeter — evolved from — Barometer. An altimeter is a barometer with an altitude scale.
2. Step 2. Barometer — works like — Manometer. Both measure pressure with a column of liquid: a barometer measures the air around us, a manometer the pressure inside a vessel.
3. Step 3. Manometer — uses — Hydraulics (Pascal’s law). A liquid manometer is a pair of communicating vessels: the difference in levels shows the pressure.
4. Step 4. Hydraulics (Pascal’s law) — is used in — Hydraulic cylinder. The liquid transmits pressure to the piston.
5. Step 5. Hydraulic cylinder — is used in — Tunnelling shield. The shield was pushed forward by jacks.

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