One of the interesting phenomena associated with hydrostatics is communicating vessels. It would seem that everything here is simple, but, nevertheless, they provide an excellent opportunity to get acquainted with the example of atmospheric pressure and plunge into the distant past.
In order to refresh the memory of what communicating vessels are, let us recall a simple experiment conducted earlier in physics classes at school. On the same plane are placed several vessels of different shapes - round, rectangular, cylindrical, in the form of a cone, and connected by a tube at the bottom level. Water begins to pour into one of these vessels, through the connecting tube water will flow into all the vessels, and, surprisingly, in all the vessels, regardless of the shape of the latter, the water is at the same level.
This is due to the fact that they are all under the same atmospheric pressure, and since they are located at the same level, then the liquid placed in them will be at the same level, because in all the vessels it is at the same pressure.
By the way, we get the simplest practical application of communicating vessels when we pour water from a teapot. While the kettle is standing level, the water level in the kettle and in its spout are the same, because the teapot and spout are interconnected vessels. The edge of the teapot spout is above the water level. If we tilt the spout of the kettle below the water level, then it starts to flow out of it.
There is a simple corollary of the foregoing. If the communicating vessels are at different heights, then pressure will act at the outlet of the tube connecting these vessels. Its value is equal to the pressure of a column of water equal to the height difference between the vessels. Everything is very simple - if the vessels are located at different heights, then water from the upper vessel will flow to the lower.
If you look at the history of technology, there are many cases when communicating vessels were used; the physics behind this phenomenon can sometimes really work wonders. How beautiful are the fountains of Peterhof! But they were built without the use of sophisticated equipment, electric motors and other machinery, which would certainly have been used by today's experts. And here, in pure form, communicating vessels are used. Ponds with water are located above the level of the fountains, which ensures the flow of water to them without any mechanisms under atmospheric pressure. It is simply beautiful, and one cannot but admire it.
Or another example, all close and understandable. Water tower. Water pumped into the tower and located at high altitude flows by gravity into the houses, and not only on the first floors. Communicating vessels again work here. Pressure, the value of which is due to the difference in height between the water tower and the water tap, will provide water to the upper floors.
Poor Romans! They did not know anything about the communicating vessels, and when they built their aqueducts to supply cities with water, they always made them with a constant decrease from the source, although in many places they could follow the topography of the soil and run pipes up the small slopes. But they always built aqueducts at a height and with a constant deviation from the source.
But the Chinese knew about the communicating vessels and, using their properties, began to build gateways. The principle of operation is very simple. Nearby are two lock chambers, interconnected by a special channel. The lock gates are closed, after which a canal is opened connecting both chambers, and water, according to the law on communicating vessels, flows to a lower level. Using a system of such locks, it was possible to carry out the movement of ships in areas with a significant difference in altitude.
Of course, the foregoing does not cover all cases of the practical use of communicating vessels, but it gives an idea of what this remarkable physical law is and how it is embodied in everyday life.