He then set out for Portugal following the 1755 Lisbon earthquake, traveling on the "Hanover", which was captured by French privateers. He was imprisoned in Brest for six days before being transferred to another prison on the French coast. He was later exchanged for a French officer held by the British, and he quickly travelled to the Commissioners of Sick and Wounded Seamen in London to seek help on behalf of his fellow captives. It is widely believed that this personal experience generated Howard's interest in prisons.