Boyle's Law with a sealed syringe

we did an experiment using a syringe that had been melted shut with a sealed mass of air inside. We hung weights from the plunger and measured the length of the air column, with got bigger as the weight increased. The cross-sectional area of the syringe was 2 square centimetres. I made a real hash of interpreting the results but I've sorted it out now. The initial air pressure inside the syringe before any weights are added is atmospheric pressure, which is 10 Newtons per square cm. As you add weights to the bottom, you are actually decreasing the pressure because as the volume increases, the number of collisions per second on the sides of the syringe decrease. The volume of the air column is given by the length of the air column x cross-sectional area. The results came out like this:

Every time, the pressure x volume comes out at a constant number, which in reality is nRT or number of moles of trapped air x molar gas constant x temperature in Kelvins. The slight variation in the constant is due to rounding errors.