Hence, when the prong is liberated, it is urged back by a force equal tothat used in detecting it. When, therefore, the prong reaches its originalposition, the force impressed during its recoil, has generated in it a correspondingamount of momentum -- an amount nearly equivalent to the force originallyimpressed (nearly, we must say, because a certain portion has gone in givingmotion to the air, and a certain other portion has been transformed intoheat). This momentum carries the prong beyond the position of rest, nearlyas far as it was originally drawn in the reverse direction; until at length,being gradually used up in producing an opposing tension among the particles,it is all lost. This opposing tension then generates a second recoil, andso on continually: the vibration eventually ceasing only because at eachmovement a certain amount of force goes in creating atmospheric and etherealundulations. Now evidently this repeated action and reaction is a consequenceof the persistence of force. The force exerted by the finger in bending theprong cannot disappear. Under what form then does it exist? It exists underthe form of that cohesive tension which it has generated among the particles.
This cohesive tension cannot cease without an equivalent result. What isits equivalent result? The momentum generated in the prong while being carriedback to its position of rest. This momentum too -- what becomes of it? Itmust either continue as momentum, or produce some correlative force of equalamount. It cannot continue as momentum, since change of place is resistedby the cohesion of the parts; and thus it gradually disappears by being transformedinto tension among these parts. This is retransformed into the equivalentmomentum; and so on continuously. If, instead of motion that is directlyantagonized by the cohesion of matter, we consider motion through space,as of a comet, the same truth presents itself under another form. Thoughwhile it is approaching the Sun no opposing force seems at work, and thereforeno cause of rhythm, yet its own accumulated momentum must eventually carrythe moving body beyond the attracting body; and so must become a force inconflict with that which generated it. This force cannot be destroyed, butit can have its direction changed by the still continued attraction: theresult being that a passage round the attracting body is followed by a retreatduring which this embodied force, gradually becoming non-apparent, is transformedinto gravitative strain, until all of it having been thus transformed therebegins a return from aphelion.
Before ending, two qualifications must be made. As the rhythm of motionitself postulates continuity of motion, it cannot be looked for when motionhas suddenly become invisible. A hint tacitly given in §82 implies thatwhat we may call a fragmentary motion -- a motion which under its perceptibleform is suddenly brought to an end -- cannot under that form exhibit rhythm: instance the stoppage of a hammer by an anvil. In such cases, however, weobserve that this non-continuous motion is transformed into motions thatare continuous and rhythmical -- the sound-waves, the ether-waves of theheat generated, and the waves of vibration sent through the mass struck: the rhythms of these motions continuing as long as the motions themselvesdo.
The other qualification is that the motions shall be those occurring withina closed system, such as is constituted by our own Sun, planets, satellites,and periodic comets. If a body approaching a centre of attraction from remotespace, has any considerable proper motion not towards that centre, this body,passing round it, may take a course which negatives return -- an hyperbola.
I say an hyperbola because the chances against a parabolic course are infinityto one.
But bearing in mind these two qualifications, of which the last may beconsidered almost nominal, we may conclude that under the conditions existingwithin our Solar System and among terrestrial phenomena, rhythm, everywherearising from the play of antagonist forces, is a corollary from the persistenceof force.
