如图1—1所示,在液体内取等高点 A 和 B,设想以 A、B 连线为轴取一横截积为 S 的圆柱体,使圆柱体的两端面分别通过 A、B 两点。令 A、B 两点的压强分别为 P_A 和 P_B,当横截面积ΔS 取得足够小时,ΔS 面上的压强就等于 A 或 B 点的压强。沿水平方向圆柱体受到周围液体的作用力分别为 As shown in Figure 1-1, take the contours A and B inside the liquid, and assume that a cylinder with a cross-sectional area S is taken as the axis A and B, so that the two end surfaces of the cylinder pass through A and B Two o’clock. Let the pressures at points A and B be P_A and P_B, respectively. When the cross-sectional area ΔS is sufficiently small, the pressure on ΔS will equal the pressure at point A or B. Cylinder along the horizontal direction by the force of the surrounding liquid, respectively