Believe I found my answer, it's the air pressure from outside hitting the mercury if they were using a Torricelli type barometer .
Thanks
Feel free to expand or edit if anyone wishes to comment
I was reading about the Pendle Hill experiment by Henry Power and Richard Towneley showing the relationship between Volume and Pressure in gas that eventually became Boyle's Law.
The higher they got the greater the volume became.
My question is, was the gas measured in the barometer isolated...