It is known that bodies in motion always describe some line or other in
the air, as the whirling round of a firebrand apparently makes a circle,
the waterfall part of a curve, the arrow and bullet, by the swiftness of
their motions, nearly a straight line; waving lines are formed by the
pleasing movement of a ship on the waves. Now, in order to obtain a just
idea of action, at the same time to be judiciously satisfied of being in
the right in what we do, let us begin with imagining a line formed in
the air by any supposed point at the end of a limb or part that is
moved, or made by the whole part or limb, or by the whole body together.
And that thus much of movements may be conceived at once is evident, on
the least recollection; for whoever has seen a fine Arabian war-horse,
unbacked and at liberty, and in a wanton trot, cannot but remember what
a large waving line his rising, and at the same time pressing forward
cuts through the air, the equal continuation of which is varied by his
curveting from side to side; whilst his long mane and tail play about in
serpentine movements.
_Hogarth._