Sol. Consider a very small volume of liquid of mass m at the surface. The adjacent layers of the liquid must exert upon this volume forces which are normal to its surface (since the liquid moves as a whole (Fig.).

If we makes this volume a thin layer. Then the forces acting on its side edges will be infinitely small and the resultant force N will be normal to the surface of the volume we have isolated for consideration. Besides the force N. its own weight mg will also act upon the volume. These two forces should impart to the isolated volume of liquid and acceleration equal to the acceleration of the vessel as it slips down the inclined plane. i.e. g sin α, where α is the angle made by the inclined plane with the horizontal. Thus the resultant of forces N and mg must be equal to mg sin α. But the force mg sin α equals the projection of mg on the line of the inclined plane. Consequently the projection of force N on to the same line must equal zero, i.e. the force N is at right angles to the inclined plane. And accordingly the surface of the liquid is parallel to the plane.