We were out on Hadrian's Wall the other month and found this water trough for cows. Our house doesn't have a water tank like this in the loft because the advent of the combi-boiler has removed the need for a hot water tank. But the ballcock is a fabulous invention. The plastic ball is less dense than water so it floats. The upthrust force on it pushes it up against the valve in the pipe, closing the valve. As the water level drops, the floating ball drops with it. Eventually there is so little upwards force on the valve that water pushed through to fill the tank. The floating ball rises to close the valve and cut off the water. An example of feedback!
