The county of Sullivan is bounded thus: Beginning at the northwest corner of Plainfield on the west bank of Connecticut river; thence easterly by the northerly lines of Plainfield, Grantham and Springfield to the westerly line of the county of Merrimack at the northeast corner of Springfield; thence southerly by the westerly line of the county of Merrimack to the northerly line of the county of Cheshire; thence westerly by the northerly line of the county of Cheshire to the northwest corner of Walpole on the west bank of Connecticut river; thence northerly up the west bank of said river to the bound first mentioned.