“Because Monsieur Eiffel is a genius,” Thomas proudly replied. “There are only a hundred and twenty of us workmen on the site at any one time. And we alone build the tower.”

Prefabrication. This was how it was done.

Out at the factory lay the network of girders, in their prefabricated sections fifteen feet long and weighing no more than three tons. Each day, the huge horse-drawn wagons would arrive at the site with just enough sections for that day’s work. Big, steam-powered cranes would lift the sections up into position, and under the watchful eye of their foreman, Jean Compagnon, Thomas and his fellow workers—the flyers, as they were proudly called—would swing their hammers onto the hot rivets to fix them in place.

“The precision is astounding,” he told his family. “Every piece fits exactly, every hole is drilled to perfection. I never have to pause in my work.” He grinned. “The whole tower will go up like clockwork. It has to,” he added. “The exhibition starts in eighteen months.”

Soon after he began work on the site, he took his brother, Luc, around it, and showed him how everything was organized. Luc was much impressed.

“And how’s your head for heights?” Luc asked him.

“No problem,” Thomas told him. “None at all.”

The foreman of the flyers, Jean Compagnon, was a sturdy workman who looked like a battle-hardened sergeant. His watchful eyes missed nothing. But Monsieur Eiffel himself was also on-site most days. Thomas took care never to interrupt the great man, but if Eiffel saw the young worker, he’d always give him a friendly nod.

As the huge lower legs began to grow, upward and inward, it appeared as if the tower’s four feet were the corners of a vast iron pyramid. Day after day the sections went in. By the end of August, the legs were over forty feet high.

Early one evening, as he was looking at the progress before going home, Thomas heard a voice at his side.

“Well, young Gascon, are you enjoying being a flyer?”

“Oh yes, Monsieur Eiffel. It’s so well organized, monsieur.”

“Thank you.” Eiffel smiled. “I’ve done my best.”

“But I suppose this is the easy part,” Thomas ventured. “When we get higher …”

“Not at all, young man. This is the hardest part, I assure you.” Eiffel pointed to the rising legs that sloped in toward the center. “Those legs are inclined at an angle of fifty-four degrees. Does anything strike you about them?”

“Well …” Thomas didn’t like to say. But the great man nodded encouragingly. “Won’t they fall over?” he finally dared to ask.

“Exactly. They will fall over, I calculate, on the tenth day of October. To be precise, when they reach a height of ninety-two feet.” He smiled. “But they will not fall over, my young friend, because we shall prop them up with big wooden pylons. You have seen the flying buttresses of Notre Dame?”