single straight line, directly toward the camera or obliquely down the hill, his movement will not be pleasing to the eye, nor will it seem natural. But if he moves in a waving line, a series of reverse curves freely made, the effect on the eye of the spectator will be somewhat like that of the "line of beauty" discussed in Chapter V.
An important difference, however, between a fixed line and one traced by a moving object is that the latter disappears as soon as it is drawn. It may linger in our memories, to be sure, yet our eyes can trace that line only once, and only in the direction taken by the moving object. That is, our physical eye cannot range back and forth over the vanished path, as it can over a fixed line. And a still greater difference is that the moving object has a rhythm of velocity as well as a rhythm of direction. Velocity and direction of movement arise and exist together, and consequently their relation to each other may produce a new rhythm. The horse, varying his pace according to the nature of the ground, may gallop along the level stretches, and may pick his way cautiously down the steep declines. There is natural harmony in rapid motion over a long smooth line, and slow motion over a short jagged one. A simple case like this may help us to answer the question, When is the relation between velocity and direction harmonious? But we have still the fundamental questions, When is a change of direction rhythmical? And, when is a change of velocity rhythmical?
We cannot promise to give direct and definite answers to these questions; but, recalling our discussion in Chapter V concerning rhythm in fixed design, let
