to Great Hale, beyond which the Main Drain was upgraded by cutting a new channel, effectively extending the South Forty-Foot Drain to Guthram, on the banks of the River Glen. A total of of highland streams were improved by scouring and raising of the banks. The estimated cost of £16,000 was raised by issuing bonds, but the project overran, and another Act of Parliament was obtained in 1770 to authorise the raising of the drainage taxes, to cover the difference. By mid-1769, when Landen and Edwards left the project, because the work was largely complete, the scheme had cost £24,000. They were replaced by Edward Hare as Surveyor of Works, with Chapman and Strattard continuing as assistants. A historian called W. H. Wheeler, who chronicled the Lincolnshire fens, wrote that "the works were efficiently carried out and, being well-designed, entirely answered expectation." On the River Witham, the Grand Sluice was constructed and opened on 15 October 1766, and this prevented tidal water from entering the river, and hence flooding the Holland Fen. The Boston Harbour Commissioners were created by the Boston Port Act 1766, and they carried out improvement works to The Haven, which resulted in lower water levels at the Black Sluice, and hence more efficient draining from the South Forty-Foot Drain. Water was pumped into the drain by a series of windmills driving scoop wheels. Maps of the area produced in 1783 by Edward Hare show 46 such mills, which provided drainage for of agricultural land. Extreme high tides in 1810 and again in 1820 resulted in widespread flooding, and further thought was given to improving the flood defences. With the improvements to the River Witham, the final section of Earl Fitzwilliam's drain to Lodewick Gowt was filled in, and the channel was diverted to join the South Forty-Foot Drain. Renamed the North Forty-Foot Drain, it now supplies Cook's Lock and Holland Fen pumping stations. Third Sluice Reports on improvements to the system were produced in 1843 by the surveyor Mr W Lewin and in 1845 by Sir John Rennie. Rennie's scheme involved the provision of a catchwater drain to collect water draining from the higher ground to the west before it entered the fenlands, but an Act of Parliament to authorise its construction was defeated, and so in 1846 a report by Mr W Cubit which proposed improvements to the existing infrastructure was accepted by the Commissioners.