This is good info on barometers 101.
I forget where I came by it, maybe this site as I have been lurking here nonstop for the past 2 weeks, filling in time at work
(1/4 of the way to cape town) waiting for some medical
problem to happen and hoping it won't.
If it is already found somewhere else on these discussion boards, my apologies.
Anyway it is from the NZ meterological site, but don't hold that against the info. In these days of complicated electronic thingys on boats, it is nice to use a piece of kit that is as simple as a wheelbarrow.
Come to think of it, it took humans 100 K years to perfect that piece of machinery. see attached pdf file