Pressure / Volume Gas relationship Pendle Hill experiment

[A Very G]

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 from the bottom of the hill as they climbed or did they repeat the experiment again at the top? If it wasn't repeated why was the gas pressure / volume level affected if it was isolated on the whole trip higher up the hill?

Thanks
Woops if this is more relevant to a chemistry forum

 

[A Very G]

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