Sail is a device used to catch the wind for use to propel a vessel through the water. In Hornblower's time is was typically made of canvas. Sails are fore-and-aft rigged and square rigged. Running before the wind, the square rigged is more efficient, whereas the fore-and-rigged can sail closer to the wind.
Names of Sails in the Hornblower universeEdit
From bottom to top.
Top Gallant (t'gallants, "t'garns'l)
Sail at Wikipedia.