Complex sentence: Hon. Mary Elizabeth Townshend, Countess Chatham (1762-1821) was the second daughter of Thomas Townshend, 1st Viscount Sydney who married General Sir John Pitt, 2nd Earl of Chatham.
Split into simpler sentences: Hon. Mary Elizabeth Townshend, Countess Chatham (1762-1821) was the second daughter of Thomas Townshend, 1st Viscount Sydney. She married General Sir John Pitt, 2nd Earl of Chatham.