Water exerts the pressure caused by its own weight and that of the air above it, equally in all directions ; and on this principle depends the hydraulic pressone of the most useful applications of a philosophical principle known in modern times. The direction of the pressure is not equal in all directions, but is controlled, in a great measure, by the shape of the containing vessel, as, for example, in a round cup having a flat bottom, the pressure is equal and greatest over the whole base, and gradually diminishes asitascendsthe sides, and so in all regular figures. In a bottle having a long narrow neck, the pressure is greatest on the base, and then on the semi-circular portion where the bottle bulges out. When constructing a canal, or water course, the sides should incline from the base outward, because then, the pressure will be at right angles with the sides, and so exert its force on the earth; whereas, should the sides be perpendicular, the pressure would be a direct thrust against it, and it would require a much stronger embankment to prevent the water forcing its way through. It is advisable also, to form the bottom inclined towards the center, or in a semicircular form. In the case of a dam to stay the course of a long current of water, or to form the head of a mill pool, the form to be preferred is a segment of a circle from side to side, and widening from the top downwards; but should the river or stream be too wide for this method to be adopted, then a straight one can be built, placed at an angle with the course of the streamlike the one on the Schuylkill, at the Fairmount Waterworksthat it may serve to break the force of the stream. If a V-shaped one be thought the best, the apex of the V must be placed against the course of the stream, and not with it; or, in other words, the outside of the letter must form the dam, and not the inside.