That little gold ball on top of the dome is an 8-foot/2 ton pair of copper hemispheres. It took Verrochio(and his young assistant, a Mr da Vinci) just 3 days to get it into place in the 15th century. Leonardo helped by making a reversible gearing mechanism for the hoists, so that horses did not need to be unhitched and made to go the other way! Bruneleschi had designed all this long before, when he was constructing the dome.