In 1729 the parish of Summersworth was organized; in 1754 this parish was erected into the town of Somersworth; in 1821 the first company was formed to develop the water-power and establish cotton and woollen mills; in 1849 the southern half of the town was setoff and incorporated as Rollinsford; and in 1893 Somersworth was chartered as a city.
In the manufacture of sugar most of the mills in use extract only about three-fourths of the juice from the cane; in 1902 about 73% of it was manufactured by 528 mills operated by steam; 17% by 470 mills operated by hand or by a carabao; and to% by 77 mills operated by water-power.
The importance of the flour manufacturing industry was originally due to the excellent water-power available, and dates from the introduction of improved roller-mill methods in the early 'seventies, although there were successful mills in operation twenty years earlier.
Water-power for factories is secured by a system of "water-power canals" from a large dam across the Savannah, built in 1847 and enlarged in 1871; the principal canal, owned by the city, is so valuable as nearly to pay the interest on the municipal debt.
In compensation the coal and gold, which form the chief mineral wealth, are found in the broken and less practicable west and centre, and these portions also furnish the water-power which may in days to come make the island a manufacturing country.