This late-Gothic tower is all that remains of the 16th-century church of Saint-Jacques-de-la-Boucherie, demolished during the Revolution. It stands alone in its small park like a misplaced chess piece — ornate, slightly brooding, and more impressive for its isolation. Blaise Pascal is said to have conducted barometric experiments from the top. Guided visits to the summit run in summer and the view over the Hôtel de Ville and the Seine is worth the climb.