CHAP. V.
Motion and Force.
THat motion is agreeable to the eye without relation to purpose or design, may appear from the amusement it gives to infants. Juvenile exercises are relished chiefly upon that account.
If to see a body in motion be agreeable, one will be apt to conclude, that to see it at rest is disagreeable. But we learn from experience, that this would be a rash conclusion. Rest is one of those circumstances that are neither agreeable nor disagreeable. It is viewed with perfect indifferency. And happy it is for mankind that the matter is so ordered. If rest were agreeable, it would disincline us to motion, by which all things are performed. If it were disagreeable, it would be a source of perpetual uneasiness; for the bulk of the things we see appear to be at rest. A similar instance of designing wisdom I have had occasion to explain, in opposing grandeur to littleness, and elevation to lowness of place[69]. Even in the simplest matters, the finger of God is conspicuous. The happy adjustment of the internal nature of man to his external circumstances, displayed in the instances here given, is indeed admirable.
Motion is certainly agreeable in all its varieties of quickness and slowness. But motion long continued admits some exceptions. That degree of continued motion which corresponds to the natural course of our perceptions, is the most agreeable[70]. The quickest motion is for an instant delightful. But it soon appears to be too rapid. It becomes painful, by forcibly accelerating the course of our perceptions. Slow continued motion becomes disagreeable for an opposite reason, that it retards the natural course of our perceptions.
There are other varieties in motion, beside quickness and slowness, that make it more or less agreeable. Regular motion is preferred before what is irregular, witness the motion of the planets in orbits nearly circular. The motion of the comets in orbits less regular, is less agreeable.
Motion uniformly accelerated, resembling an ascending series of numbers, is more agreeable than when uniformly retarded. Motion upward is agreeable by the elevation of the moving body. What then shall we say of downward motion regularly accelerated by the force of gravity, compared with upward motion regularly retarded by the same force? Which of these is the most agreeable? This question is not easily solved.
Motion in a straight line is no doubt agreeable. But we prefer undulating motion, as of waves, of a flame, of a ship under sail. Such motion is more free, and also more natural. Hence the beauty of a serpentine river.
The easy and sliding motion of fluids, from the lubricity and incoherence of their parts, is agreeable upon that account. But the agreeableness chiefly depends upon the following circumstance, that the motion is perceived, not as of one body, but as of an endless number moving together with order and regularity. Poets struck with this beauty, draw more images from fluids than from solids.
Force is of two kinds; one quiescent, and one exerted by motion. The former, dead weight for example, must be laid aside; for a body at rest is not by that circumstance either agreeable or disagreeable. Moving force only belongs to the present subject; and though it is not separable from motion, yet by the power of abstraction, either of them may be considered independent of the other. Both of them are agreeable, because both of them include activity. It is agreeable to see a thing move: to see it moved, as when it is dragged or pushed along, is neither agreeable nor disagreeable, more than when at rest. It is agreeable to see a thing exert force; but it makes not the thing either agreeable or disagreeable, to see force exerted upon it.
Though motion and force are each of them agreeable, the impressions they make are different. This difference, clearly felt, is not easily described. All we can say is, that the emotion raised by a moving body, resembles its cause: it feels as if the mind were carried along. The emotion raised by force exerted, resembles also its cause: it feels as if force were exerted within the mind.