Vernier Barometer at Eastbourne Lifeboat

 

I've never seen a Vernier on a mercury barometer but I found this in the Eastbourne lifeboat shop. Bottom of the sliding scale shows you where to read on the fixed scale (red arrow 30.3) Then find where the lines on the two scales match up and take the number from the sliding scale (blue arrow 2). Put them together to get 30.32.