Who is Admiral the Honorable Sir William Cornwallis from Hornblower and the Hotspur and what is their importance?

Cornwallis assigns Hornblower command of the Hotspur prior to England's war with France. He is a senior officer, who is expected to choose the right men to command the fleet, and renown for for his ability to do just that. At the end of the novel, Hornblower is promoted at Cornwallis' suggestion.