Incline very gently until nearly inverted, when
Air if present will ascend in a bubble into the cistern.
Suspend barometer in good light out of sunshine.
Let no heat of fire or lamp affect it.
Let no sudden changes of temperature affect it.
It must hang absolutely vertically.
Note temperature of attached thermometer before reading barometer.
Then adjust mercury in cistern to touch ivory point.
Then adjust vernier and take reading quickly.
Ascertain height above sea-level according to direction.