Armstrong was also the driving force behind the Whittle Dean Water Company which was formed in 1845 to supply Newcastle and Gateshead with a fresh water supply from newly-created reservoirs at Whittle Dean, about a mile north west of the village of Horsley in Northumberland.
From there, the water would flow along pipes through Wylam, Newburn and Lemington to Newcastle and Gateshead. Water would be supplied directly to every house and building, dispensing with the need for pumps and communal stand-pipes and ensuring a supply of water to every house. Armstrong became Secretary to the company and Donkin, Stable & Armstrong acted as its solicitors.
The Company's prospectus was published early in 1845 with the purpose of attracting potential shareholders. In addition to outlining the way in which the proposed system would work, the Prospectus also listed its potential benefits; in addition to the relief of poor families from carrying water at a distance, the Company asserted that the new water supply would have the added advantage of being:
the means of preventing those promiscuous assemblages of young persons, while waiting for the water where it is sold, of which the demoralising tendency is severely commented upon in the evidence published by the Health of Towns Commission. 1