In 1652, he left on a leave of absence and travelled with Oliver Cromwell's army in Ireland, as physician-general. His opposition to conventional universities, being committed to 'new science' as inspired by Francis Bacon and imparted by his afore-mentioned acquaintances, perhaps pushed him from Oxford. He was pulled to Ireland perhaps by sense of ambition and desire for wealth and power. His breadth of interests was such that he successfully secured the contract for charting Ireland in 1654, so that those who had lent funds to Cromwell's army might be repaid in land – a means of ensuring the army was self-financing. This enormous task he completed in 1656 and became known as the Down Survey, later published (1685) as "Hiberniae Delineatio". As his reward, he acquired approximately in Kenmare, in southwest Ireland, and £9,000. This personal gain to Petty led to persistent court cases on charges of bribery and breach of trust, until his death.