21. The U-tube manometer poured mercury. Open knee manometer is connected to the surrounding space at atmospheric pressure ρ0, and mercury in an open knee stands higher than the closed at Δh = 10 cm. The free part of the mercury tube closed knee has a length l = 20 cm. When the open knee attached to the balloon with air, the difference in mercury levels increased and reached a value Δh1 = 26 cm. Find the air pressure p in the cylinder.