Place whose Visibility Changes with Tide

Problem

 * There is a certain place in, where, when the tide is in,
 * the sheep may be seen feeding on a certain neighbouring island;
 * yet, when the tide is out, and the water at the lowest,
 * not one can be seen, though they be feeding there at the same instant.

Solution

 * The place may be the wharf at, the over against it,
 * and the appearance caused by refraction, when the water is high.