A marot jar is a special kind of syphon or siphon device invented some time during the 1700s. The device is filled up water and lets it out in a special way so that the speed and amount of water that comes out is always the same, even if the jar is completely full or almost empty.
In the diagram (see right), an air-tight jar contains some water (blue) and some air (yellow). The green pipe is started as a siphon so that water starts flowing out of the pipe. Because the pipe lets out water, the pressure on the inside of the jar is less than the pressure outside, so it causes air to be sucked in through the orange pipe. Because the pressure is going to tend to being the same inside and outside of the jar, the water is going to flow out of the pipe at the same speed and amount.