Water exerts a pressure on the bottom and sides of the vessel which holds it. Fill a vessel 1 cu. ft. in volume with water. If the water is weighed it is found to weigh about 62.5 lbs. Therefore 62.5 lbs. is pressing on the bottom of the box, the area of which is 144 sq. in. Therefore the pressure per square inch is 62.5 / 144 or .434 lb. The unit of pressure is the amount of pressure to the square inch. Pressure equals force per unit area.

A liquid also exerts pressure on the outside of any object immersed or pushed into it and the pressure increases with the depth. This phenomenon may be explained by considering a liquid as made up of a large number of thin horizontal layers, each layer supporting the weight of those above. The lower the layer, the greater the weight of liquid it has to support; hence the greater the pressure exerted upon it. This pressure has nothing to do with the size and shape of the vessel and is evenly exerted upon each square inch of surface.

The total pressure of a liquid upon any portion of the vertical sides of a vessel is equal to the weight of a column of the liquid, whose base and length are respectively the area of that portion of the side and its average depth. This may be explained in another way. The pressure against the vertical side of a tank at the surface of the water is zero, for the liquid has no depth. But the pressure on the side increases with the depth until we reach the bottom of the tank, when it is equal to the pressure against the bottom.

The average pressure on the side then is the pressure exerted on the middle of the side, and is equal to one-half the pressure per unit of surface against the bottom.

The following laws apply to liquids:

I. The pressure does not depend upon the size or shape of the vessel. The pressure increases with the vertical depth below the free surface.

II. At any point in a liquid, the upward, downward, and lateral or sideways pressures are equal.

III. To find the lateral pressure of water, upon the sides of a tank, multiply the area of the submerged portion of the side in inches, by the pressure of one-half the depth.

As an example: What is the lateral pressure on one side of a tank 20 in. wide and 2 ft. deep (Fig. 42)? The solution is as follows: 