Mercury Barometer Experiment: Atmospheric Pressure

[ftball22]

Homework Statement

Hi guys, I'm new here and I hope you can help me with this question that's been puzzling me for a good week now.

Mercury barometer of 1m with mercury column height of 760cm is equivalent to 1.0x10^5 Pa.
School experiment to find atmospheric pressure was as follows. Length of air trapped in capillary tube sealed at one end by thread of mercury. When horizontal, the length of trapped air column was 82mm and length of thread of mercury was 39mm. When tube held vertically with open end upwards, air column was squashed to 78mm.

Find value of atmospheric pressure in mm of mercury mmHg

Homework Equations

Not sure if there is one
Does P=F/A help?

The Attempt at a Solution

To be honest I really haven't had much to go in, but I can only think that gravity must be playing a part in it squashing the mercury down when it is vertical?

Any help appreciated!

 

[rude man]

Hint: consider using Boyle's law.