It is Thermo-siphon type cooling system. As the water heats up it rises to the top of the engine and out the top hose into the raditor where the fan cools it down and it falls to the bottom of the raditor and back into the engine. This system actually works quite well as long as the system it clean, not full of rust and gunk build up.
No water pump, No thermostat, NO PROBLEM!!!!!!!!!!
Premix your anti-freeze and water to insure a good mix.