Pressure in the air inside the column of mercury is equal to the weight of mercury over the air divided by the internal cross sectional area of the tube. When the temperature increases, the weight of the upper part of the mercury column does not change. That is why the pressure in the air is also constant. for the isobaric process, the change in volume is propotional to the change in temperature. The same is true for the lengths of the air column.
`(l)/(l_(0))=(T)/(T_(0))rArrl=(l_(0)T)/(T_(0))=11`