The tramway to Silkstone was completed in 1810. Barnby basin was enlarged to cope with the expected trade, which grew steadily. Coal accounted for just 23 per cent of the 22,270 tons carried in 1800, but had risen to 109,945 tons out of a total of 181,063 tons by 1821. The other main cargoes were corn and limestone. Receipts also rose, from £2,028 in 1803 to 7,649 in 1821, the latter figure being supplemented by a further £2,094 raised from tramway tolls. Profits were sufficient to pay dividends from 1810. In order to keep the boats moving, the Barnsley Canal Company attempted to take water from the River Dearne on several occasions, and were ordered to remove the works which diverted the water by the River Don Navigation Company, who had sponsored the Dearne and Dove Canal. A compromise was reached in 1812, which allowed some of the Dearne water to be used by the Barnsley canal. Between 1828 and 1830, a number of bridges were raised, to allow larger vessels, called "Billy-boys", to use the canal.
Question: Across how many years were bridges raised to allow "Billy-boys" to use the canal?
Answer: 2