To double the force on your whole head, we d have to cover your entire head with a column of mercury 76 cm tall at every point. If we did that, we d increase the pressure on your entire head. Every place on your head, regardless of size, would feel the same pressure. Your nose would feel the same pressure as your ear lobe. Therein lies the difference between weight and pressure.
Pressure is what held the water and mercury up in our experimental tubes.
It didn t matter what size bucket we used. The pressure on the surface of the mercury is the same. At that point the pressure, independent of the diameter of the tube , due to the weight of mercury in the column is equal to the atmospheric pressure. You may sense some circular reasoning here. He noticed that gases are compressible.