N.H. Rev. Stat. Ann. § 22:2

The county of Rockingham is bounded thus: Beginning at the mouth of Piscataqua river and running up the same to the easterly corner of Newmarket, including the river; thence northwesterly by the easterly and northerly lines of Newmarket, Epping, Nottingham and Northwood to the easterly line of Pittsfield; thence southwesterly by the northerly and westerly lines of Northwood, Deerfield, Candia, Auburn and Londonderry to the northerly line of Hudson; thence by the northerly and easterly lines of Hudson, to the northwest corner of Pelham; thence by the northerly line of Pelham to the state line; thence by the state line to the sea; thence by the sea to the bound first mentioned, including all of the Isles of Shoals which belong to this state.