In 1761 Desaguliers was summoned from his experiments and manufactures to take command of the siege train and the force of artillerymen intended to accompany the expedition to the island of Belleisle, off the western coast of France. This was the first opportunity to test on a large scale the improvements made in siege artillery since the days of Marlborough, and Desaguliers was able to put his ideas into practice. General Studholme Hodgson was in command, with Generals John Crauford, William Howe, John Burgoyne, and Guy Carleton under him. When Desaguliers arrived at Belleisle on 12 April with the temporary rank of brigadier-general, one unsuccessful attempt had already been made to disembark. Desaguliers at once volunteered to reconnoitre, and by putting some of his heavy guns into ships' boats, managed to cover the landing of the army. The island soon submitted, and General Hodgson directed Desaguliers to form the siege of the citadel. The manuscript journal which he kept during the siege of all his operations is preserved in the Royal Artillery Institution's Library at Woolwich, and forms the basis of the account given of the siege by Francis Duncan in his "History of the Royal Regiment of Artillery". Desaguliers got thirty guns and thirty mortars into battery, fired 17,000 shot and 12,000 shells into the citadel. He had great difficulties to contend with owing to the flooding of the trenches, and was wounded himself five days before the capitulation of the fortress on 7 June.