siphon is the answer.
placed a pipe into the upper barrel, partially filled with liquid.
The pipe was then extended down to a lower barrel. To start the
siphon, suction was needed at the bottom of the tube. A partial
vacuum is formed at the bottom which allows atmospheric pressure
to force water along the tube. This causes water to flow up the
tube, over the bend (X), and along the tube towards the lower barrel.
the water reaches the same level as the end of the tube in the top
barrel (point Y) the mass of water descending the pipe continues
to draw more water from the upper container unaided. The longer
the pipe descends from point Y, the faster the water will flow through
the tube. The most important thing is that the rate at which the
water flows is independent of the volume of water in the top barrel.
It flows to the lower barrel at the same speed whether the top barrel
is nearly full or almost empty.