I wouldn't bother trying to fix it. Just be careful. Remember, it's actually the vapour given off by mercury that will send you mental. Also, don't use any aluminium tools or anything aluminium near it. Mercury is a bioaccumulative toxicant that 's easily absorbed through the skin, respiratory and gastrointestinal tissues.
I use a mercury thermometers at work and had the same problem with bubbles in the mercury column - the way in which I got it fixed was to freeze it down to a temperature where all the mercury "shrinks" back inside the bulb at the base - don't know if it wold work on a barometer though.
Please be careful subjecting a thermometer to heat ! Overdo it and it will, without warning, explode like a high velocity misile. Shards of glass and mercury at the speed of sound, or is it above? - anway not recomended.
Just felt I should add that two pe'orth for anyone just in case.
Thank you for the suggestions, I should have mentioned that the barometer is of the 'stick' kind, around 250 years old and about 3' long...
Big microwave ;-)
I wondered if turning it upside down would work. Any ideas?
Don't turn it upside down you could lose all the mercury. with thermometers you can swing them round sharply. Don't know if it will work with a barometer though.