Quote: "When I read your initial question, TuxfordC, I was glad to hear the answer to an itch at the back of mind - how some mercurial barometers can be recorded without any attached thermometer."
Barometers were around for a while before folk started thinking about temperature corrections. There another factor too.
We/they are supposed to be reading the height of a column of mercury that indicates the pressure of the air. At the bottom of every
practical barometer is a reservoir that stores the mercury that's not in the 30 inch tube. As the height of the tube column
varies, so does the height of the mercury in the reservoir. The Fortin pattern barometers make allowance for this by making the
reservoir partly of leather which can be squeezed by an adjusting screw to vary the height of mercury in the reservoir. There is
a fixed marker pointer, once made of ivory, and the observer has to adjust the reservoir until the marker is just touching the surface
of reservoir mercury. Then, the reading at the top of the column is correctly showing the height of that column and hence the pressure.
I found it hard to enough to correctly adjust the pointer when on stable Mother Earth (according to my Tutor) so can imagine the
difficultly of using a Fortin pattern on a pitching and rolling vessel. Perhaps they did not? Certainly barometers are mounted in
gymbals, even on land, but the only ones I have read at sea were aneroid pattern relying on an accurate metal vacuum box rather
than a column of mercury.
PS Thankfully they used mercury instead of water. A water barometer is about 30 feet.One could be taking baro' readings up in the tops'ls.