If Torricelli's had drilled a little hole at the top oh the glass tube holding his mercury column what would have happend to his barometer?

If Torricelli had drilled a hole at the top of the glass tube holding the Mercury column, the mercury would flow out of the tube due to the air pressure pushing down on the surface of the mercury. This would cause the column to drop, and the barometer would no longer be able to accurately measure atmospheric pressure.