The third property in such objects is, that though the surface continually varies its direction, it never varies it suddenly.

The application of anything sudden, even though the impression itself have little or nothing of violence, is disagreeable.

The quick application of a finger a little warmer or colder than usual, without notice, makes us start; a slight tap on the shoulder, not expected, has the same effect.

Hence it is that angular bodies, bodies that suddenly vary the direction of the outline, afford so little pleasure to the feeling.

Every such change is a sort of climbing or falling in miniature; so that squares, triangles, and other angular figures are neither beautiful to the sight nor feeling.