Figure-1 shows a length of capillary tubing in which a column of air is trapped by a mercury column of length 100 mm. The length of the air column is 400 mm. The bottom of the tubing is sealed and the top is open to the atmosphere. The tubing is now inverted, figure-2, and the air column is seen to increase in length to 500 mm. Use this observation to calculate a value for atmospheric pressure, expressed in $\frac{\mathrm{mm}\mathrm{of}\mathrm{mercury}}{100}$.