Complex sentence: Returning to Lowell he was again engaged with Mr Francis in hydraulic work and the construction of the lower end of the Northern Canal he then designed and built a dam and the mills on the Suncook River at Suncook, New Hampshire.
Split into simpler sentences: Returning to Lowell he was again engaged with Mr Francis in hydraulic work and the construction of the lower end of the Northern Canal. He then designed and built a dam and the mills on the Suncook River at Suncook, New Hampshire.