The average momentum of the molecules in the direction along the surface will not change during their impact with the surface. Furthermore, since the momentum of the molecules is randomly distributed in space, the magnitude of the force due to pressure should be independent of the direction of the surface on which it acts. The vector force due to pressure acting on an element of surface. An important property of pressure is that it is transmitted through the fluid. When an inflated bicycle tube is pressed at one point, for example, the pressure increases at every other point in the tube.
Measurements show that the increase is the same at every point and equal to the applied pressure.
For example, if an extra pressure of 5 psi were suddenly applied at the tube valve, the pressure would increase at every point of the tube by exactly this amount. This property of transmitting pressure undiminished is a well established experimental fact, and it is a property possessed by all fluids. The transmission does not occur instantaneously, but at a rate that depends on the speed of sound in the medium and the shape of the container. The shape of the container is important because pressure waves refract and reflect of the walls of the container and this increases the distance and time the pressure waves need to travel. This phenomenon should be familiar to anyone who has experienced the imperfect acoustics of a poorly designed concert hall.