The following is the entry for Thomas Alexander Pakenham in William O'Byrnes 1849 'Naval Biographical Dictionary'.
PAKENHAM. (Lieutenant, 1844.)The Honourable Thomas Alexander Pakenham, born 3 March, 1820, is third son of the late Earl of Longford, by Georgiana Emma Charlotte, daughter of William, first Earl of Beauchamp. His eldest brother, the present Earl of Longford, is an officer in the Second Life Guards; and three other of his brothers are also in the army.
This officer entered the Navy, from the Royal Naval College, 14 April, 1835; passed his examination 2 May, 1840; was for some time attached, as Mate, to the Excellent gunnery-ship at Portsmouth, Capt. Sir Thos. Hastings; and, after a servitude of nearly three years in that capacity on board the Warspite 50, Capts. Lord John Hay and Provo Wm. Parry Wallis, on the North America and West India and Mediterranean stations, was promoted to the rank of Lieutenant 10 June, 1844. His appointments have since been – 9 Sept. 1844, to the Mutine 12, Capt. Rich. Borough Crawford, at the Cape of Good Hope – and, 2 Sept. 1845, to the President 50,. flag-ship of Rear-Admiral Jas. Rich. Dacres, on the same station, where he is now employed.