To study the relation between volume and pressure of air

Get  a rubber bung or 'door stop' which just fits in a narrow glass jar or measuring cylinder. Attach it  to  the  lower  end  of  a wooden rod. Fit a tin lid to the upper end of the rod to act as a scale pan. Lubricate the pistons makes with a little vaseline or heavy engine oil. Use the piston to trap air in the jar; put dissimilar weights on the pan and calculate the volume of air in the glass cylinder for each weight. Note that the volume is in inverse proportion to the pressure.