- [Narrator] 1,000 feet tall.

Today, there are countless structures that rise higher, but at the time, 1,000 foot tower made of iron seemed like utter fantasy.

- [Translator] At the time of the Eiffel Tower Project, the highest structure was the Washington Monument, which is half the size of the Eiffel Tower, and built in masonry.

That construction took several decades as the ground underneath kept sinking.

Eiffel's idea was to use a completely different material to switch to metals, which solved the weight issue, but then they faced a new problem, wind.

- [Narrator] As a tapered vertical structure, 1000 feet tall, the design had to withstand variable wind speeds that would change at each level along its height.

Those winds were not well understood, and when calculating the tower's aerodynamics, Eiffel and his engineers only had theory to guide them.

Here, physicist Benoit Roman compares the effects of wind on two models, a straight tower on one side, and the Eiffel Tower on the other.

- [Translator] So here we have a wind speed of 10 miles per hour.

We see very clearly that the straight tower is bending much more than the Eiffel Tower, which shows its higher rigidity and wind resistance, even though they're the same height and have the same quantity of materials.

- [Narrator] So why does the Eiffel Tower resist wind so much more effectively?

Iron is a flexible material, so the great height of the structure makes it vulnerable to large wind forces.

But with this unusual shape, the force of the wind and of the tower's own weight naturally directs the resulting force downward, following the curves of the tower.

- [Translator] This is the best shape imaginable for wind resistance.

It has the elegance of a mathematical solution.

It's truly optimal.