"Hence a ship is said to be tight, when her planks are so compact and solid as to prevent the entrance of the water in which she is immersed: and a cask is called tight, when the staves are so close that none of the liquid contained therein can issue through or between them"

This quote is referring to the principle of tightness in relation to ships and casks. A ship is said to be tight when its planks are so close together that water can not enter the ship. Likewise, a cask is said to be tight when the staves are so close together that none of the liquid inside can escape. This concept of tightness is important for both ships and casks, as it ensures that the ship or cask can stay watertight and the contents stay safe. Without tightness, ships and casks would be unable to work correctly and would be at danger of handling water or dripping their contents. This quote by William Falconer stresses the significance of tightness in both ships and casks.