Map courtesy Tailte Éireann/OSI
If
you look at the old OSI maps, you may have noticed H. Ram or "hydraulic
ram" indicated on some of them. A hydraulic ram is a type of cyclic
pump, operated by hydropower which has been in use for centuries. The
device doesn't need any electricity to power it and before the invention
of solar panels, was useful for pumping water in remote areas. A
hydraulic ram uses the water hammer effect and kinetic energy of water
as a power source, and converts high flow, low pressure head into lower flow, higher pressure head, allowing water to be pumped to a higher level.
More information on how hydraulic rams work in this Wikipedia article. https://en.wikipedia.org/wiki/Hydraulic_ram
In
fluid mechanics, pressure head is the height of a column of water above
a certain point. So the cold water tank in your attic has a pressure
head of 16 feet/5 m or so, creating pressure for the hot water tank if
it's on the ground floor.