During the Middle Ages and industrial revolution the area which now forms The Downs was used as common grazing land. It was mined for lead, calamine, iron and limestone, and became home to a windmill which produced snuff from the tobacco which had become one of the city's principal imports. In 1777 the windmill burnt out in a storm, and the building was converted into the observatory, which houses a camera obscura. In the 18th and 19th centuries Bristol's economy boomed and Clifton became a desirable place to live. Mansion houses were built overlooking the gorge, but after grazing was stopped, trees grew and obscured the view from these mansions. In the Victorian era, with houses creeping further onto the Downs, an Act of Parliament was passed to protect them as a park for the people of Bristol. In 1754 a bridge to span the gorge was proposed, but it was nearly 80 years before work began on Isambard Kingdom Brunel's Clifton Suspension Bridge, and a further 30 years before it was completed. Today the bridge is perhaps the best known landmark in Bristol.