Leaning tower of Pisa. Engraving of the leaning bell tower, or campanile, of the cathedral of the city of Pisa, Italy. Construction began on it in 1173 but was halted when it began to subside after the first three stages had been built. Construct- ion began again in 1275 when attempts were made to correct the lean. By 1301 six stages were built and the uppermost part was completed in 1372. The tower is 55 metres tall, 16 metres in diameter, and at its summit leans 5v metres from the vertical. It is thought that Galileo (1564-1642) may have performed gravity experiments by dropp- ing weights from the topmost stage to show that their time of fall was independent of their mass.