Torrento

Meet Torrento, a water tank who sends out a faster jet the deeper she is filled.

Evangelista Torricelli found in 1643 that water flowing out of a small hole in a tank leaves at the same speed it would reach by falling freely from the water's surface down to the hole. The speed depends only on the depth h below the surface.

For a hole 0.80 m below the surface, the speed is v = √(2 × 9.81 × 0.80) ≈ 3.96 m/s. To double the speed, the depth has to be four times greater.

The law can be derived from Bernoulli's equation, which came almost a century later. As water leaves the hole, pressure energy from the water above turns into kinetic energy. The shape of the tank does not matter, only the height of water above the hole.

Real jets are a little slower because of viscosity, and the stream narrows just after leaving the hole. As the tank drains, h falls, so the jet slows and the flow gets weaker. This is why a draining bath empties more slowly toward the end.