A Declaration Concerning the newly invented Art of Double Writing was a pamphlet of 6 leaves, written by Sir William Petty (1623-1687) and first published in 1648. It contained information regarding his invention of the "Art of Double Writing", especially a claim for patent rights. It did not contain any information on what his invention exactly was. Petty went to Ireland in 1652 to assist Oliver Cromwell in his conquest and arranged the Down Survey, a cadastral survey of Ireland, between 1654 and 1656. In 1662 Petty was one of the founding members of the Royal Society.