THOUGHTS ON THE NATURE AND ORIGIN OF FORCE.

BY WILLIAM B. TAYLOR, OF WASHINGTON.

The great generalization of our age-the continuity or indestructibility of force-confirmed as it is by a constantly accumulating series of inductions, contradicted as it is by no adverse fact, comes gradually to impress itself upon our convictions almost with the positiveness of an axiom. And yet a slight consideration will show us that it is really but a corollary from the particular constitution of matter presented constantly to our observation. Had we any example of an incompressible substance wholly devoid of elasticity—that is, possessing no molecular or atomic repulsion, (to adopt the conception entertained by Newton and his followers,) but endowed with only cohesive and gravitative attractionsthen for such matter the theory of the conservation of force would not be true. Were two equal balls of such a material to meet in space with equal and opposite velocities, at the moment of impact there would be, with the arrest of motion, the actual destruction of the entire living force they jointly possessed. Not only would there be no rebound, but there would be no internal molecular work effected, and no heat generated to represent a surviving or transformed energy.

The doctrine of the correlation and conservation of forces depends therefore upon the coexistence in all matter of two opposing tendencies, namely, those of attraction and repulsion; and from the ceaseless play and struggle of these opposing tendencies are derived ultimately all the varied phenomena of the visible universe.

The most ordinary and obvious manifestations of force are those exhibiting motion; as in the whirlwind and the cataract, the volcano and the earthquake, the steamship and the locomotive train. There has accordingly been a very general tendency to confound force and motion; and this fallacious confusion has not unfrequently vitiated the reasonings of even intelligent writers.

To distinguish, however, motion from force on the one hand, it may be remarked that they never have a common measure; and to distinguish force from motion on the other hand, it is only necessary to consider the fact that we are surrounded by a vast array of statical forces, continually resisting the most energetic solicitations to motion.

First, as to the absence of a common measure: It is found that the rate of motion follows only the law of the square root of the force originating it. A double consumption of fuel will not double the speed of a locomotive engine. On the contrary, (as is well known to the engineer,) four times the quantity of fuel is necessary to attain this duplication of

effect. So if a pound of gunpowder will impel a cannon-ball with a given velocity, it will require four pounds of powder to double that velocity. The conservation of force, however, is maintained in the fact that the penetrating power of the cannon-ball is directly proportional to the energy expended in its propulsion; or, in other words, to the square of its velocity. Or, to express the distinction in the established formula, while mv represents the quantity of motion in a moving body, m × v2 represents its quantity of force.

Secondly, in regard to static force: Every one who has ever attempted to hold aloft a heavy weight as motionless as possible will have had a realizing sense of the expenditure of energy required, not to produce, but to prevent motion. The Suspension Bridge at Niagara, safely upholding its thousands of tons of loaded cars and human freight over the frightful chasm beneath, may be cited as one among an infinite number of examples of statical force, or of power in repose.

The avalanche, hurtling down the mountain side with destructive violence, overwhelming a village and its inhabitants, is but expending a force stored up a year or years before, by the sun, when it lifted the mass, molecule by molecule, to its position of latent or potential energy. Every pound of coal possesses a static or potential force of ten million "foot-pounds." That is to say, the power expended by the sun in rais ing the pound of coal from its low estate of chemical combination, or a satisfied affinity, to the higher plane of isolation and capacity for chemical reunion, was a power capable of lifting one thousand pounds of water to the height of ten thousand feet. And, conversely, the pound of coal thus chemically raised has itself received the power of mechanically lifting that immense weight to that enormous height.

It must be borne in mind, however, that when the sun actually does lift a thousand pounds of water to the height mentioned, it does not raise it bodily as water; it performs the vast additional labor of tearing asunder the entire mass, molecule by molecule, in opposition to the statical force of an intense cohesive attraction: a work of more than ten thousand "foot-pounds;" an expenditure of energy greater than that required to grind the same weight of ice (one thousand pounds) to the most impalpable powder. Deducting, therefore, this expenditure (a little over ten thousand "foot-pounds ") we should find that the pound of coal represents, in a static form, an invested power equivalent to the lifting of one thousand pounds of water a little more than one foot, in addition to the task of evaporating the whole amount.*

Matter in motion being thus merely a vehicle of force, it follows that wherever dynamic or kinetic energy is transformed into potential energy, (as in pumping up water into an elevated reservoir,) there motion is to this extent destroyed; wherever potential or static energy is transformed into dynamic, (as in the discharge of a loaded pistol,) there motion is

*This of course does not include any actual heating of the water. One pound of coal will raise thirteen thousand pounds of water 1°; which is equivalent to only seventy-two pounds of ice-cold water (32°) raised to the boiling point, (212°.)

created. A very striking example of the continual transformation back and forth of these two conditions of force is presented in planetary movements, especially in those of considerable eccentricity. Taking the case of a cometary orbit, for example, whose greatest elongation should extend beyond the distance of Neptune, while its perihelion should lie far within the orbit of Mercury, we should find that the body, rushing or falling toward the sun with accelerated rapidity, would finally acquire a velocity of a hundred or two hundred miles per second; when its accumulated momentum would suffice to hurl it off as a prôjectile to its remote aphelion, where its velocity would be reduced to two or three miles per second, and the body be again in a condition to repeat its mighty oscillation. And this majestic pendulum, occupying some three-quarters of a century in its excursion, while exhibiting successively so enormous an absorption and alternate generation of motion, would at the same time illustrate the constancy and indestructibility of its force, in its transformation from the latent to the actual, and vice versa; the vis-viva of its lowest speed, plus its co-existing potential of gravitation at highest elevation, being exactly equal to those of its highest speed and lowest fall.

And yet, obvious as these truths appear, an acute and comprehensive thinker in an admirable exposition of the doctrine of evolution, has devoted a chapter to the unfortunate fallacy of the "Continuity [or indestructibility] of Motion." If it be once admitted that motion can be transmuted into any other form of force, then of necessity it cannot exist also as motion. The loose brick balanced on the chimney top, ready at a sudden gust of wind to topple and to strike a passer-by to earth, has a potential force, imparted to it perhaps fifty years ago by the hod-carrier and the brick-layer who raised it and placed it in its seat of power. During all these fifty years it has lain there quiet. True, it has partaken of the earth's rotations and revolutions; true, it has responded faithfully to all the varying, never ceasing vibrations of summer-heats and winter-colds; and in all this passive acquiescence it has but illustrated its inertia. But this is manifestly altogether foreign to what we are considering. Whence, then, the motion that, commencing with the fall from the chimney, hurled back the brick to the earth from which it had so long before been lifted? The blast that tilted it was but the trigger which set free its latent power. Shall we say that the original motion imparted to it was also latent during all these years? Such an expression as "latent motion " is but a senseless contradiction of terms. The cap-stone of the great pyramid, elevated to its dizzy height with much effort and labor, might possibly remain there unmoved forever.

The simple truth is, that so far from our having any warrant for the assumption that the sum of all the motions in the universe is a constant, the repeated creation and destruction of motion is on the contrary necessarily involved in the indestructible transmutability of force.

These considerations, of course, apply equally to all the various kinds of molecular movement, such as those of light, of heat, and of electrical currents. It is familiar to all that we can create heat, as when we kindle a fire in the stove, or pass an electrical current through an imperfect conductor, or simply rub two sticks together. It is equally familiar that we can destroy heat, as when we employ steam-power for welding, forging, rolling, or swaging iron, and measure the effective work per formed in these operations by the amount of heat abstracted from the steam, and forever destroyed as heat. In brief, whenever heat or other movement has produced a changed effect in matter, either internally or externally, there and to that precise extent has the motion (whether molar or molecular) entirely disappeared.

It is true that we still hear the convenient term "latent heat" frequently employed; but while holding in all honor the researches of Black, who first unfolded to us the curious phenomena grouped under this title, we now know by the clear light of the dynamic theory that there is no such thing as latent heat; that if heat be not sensible, or actual, it is not heat at all. We now know that the 142° necessary to liquefy melting ice without any increase of temperature, and the 965° absorbed in effecting the evaporation of boiling water, have had to overcome great molecular resistances; and that the internal work thus performed in raising the water to a higher potential is exactly measured by the amounts of heat respectively thus expended and consumed. When, true to the eternal law of conservation, precisely similar amounts of heat are obtained by a reversal of the several processes, these temperatures are as much a new creation or transformation as when we ignite the carbureted hydrogen at our gas-burners, or the anthracite in our grates.

No one would think of saying (excepting metaphorically) that we were releasing the light and heat stored up-in the one case at the retorts of the gas-factory, and in the other in the carboniferous laboratory of the solar actinism-a million years ago. Heat latent in coalfields which may perchance have lain immediately beneath colossal glaciers for thousands of years! As well might we speak of the blaze emitted by the petroleum lamp as original sun-light which has been latent all these millenniums.

Undoubtedly the more accurate designation of the fact is, that when motion has resulted in static condition, the motion is absolutely destroyed; when from that condition a succeeding motion is evolved, a new motion has been as absolutely created; that when one kind of motion has been transformed into another kind of motion, the equivalence of the two by no means involves the identity of the two.

Among the multitudinous metamorphoses of force presented to our observation, we find not unfrequent examples of motion rising higher than its source. These cases may, however, all be likened to the familiar illustration of a large weight on the short end of a lever lifting a lighter weight on the other end as many times as high; or to the par

allel instance of a considerable body of water by its momentum raising a smaller quantity to a higher level in the hydraulic ram. Similar instances of water-power occur in nature, as at the mouths of rivers, and at estuaries, where the contracting channel transforms a large mass movement into the potential of elevation. A magnificent example of this is furnished in the Bay of Fundy, where a huge tide of seventy feet is derived from an ocean-wave probably not exceeding two feet in height. As in visible or mass motion, the matter may be either falling to a lower state of power, (as in the descending weight of a clock or the falling water of Niagara,) or rising under a superior external propulsion to a higher state of power, (as in a clock being wound up or ocean-water being, evaporated by the sun,) so in molecular movements, the matter may be either running down, (as in combustion, or in the decomposition of quaternary or ternary compounds into more stable binary compounds,) or the matter may be raised to higher power, (as in the vegetable deoxidation of carbon by the sun's rays, or in the building up of animal substance to more complex and unstable conditions, by the power derived from other matter running down.)

Now, in all these wondrously varied and complex transmutations of force, while it is certain that the sum of all the static and kinetic forms of energy in the universe is a constant, which the human race with all its endless appliances of machinery can no more increase or diminish than it can add to or subtract from the quantity of matter, yet it is equally true that the store of the former, or the potential in Nature, is being in the aggregate diminished by transfer to the condition of the latter or the dynamic; and that in this transfer the general tendency is to a form of temperature of greater diffusion, and less capability of further transformation. So that solar and planetary systems are constantly running down to a lower plane of power-the former by the radiation of high heat, the latter by the radiation of low heat-into empty celestial spaces. There is on the whole, therefore, as Professor Sir William Thomson has well designated it, a "Dissipation of Energy."

In a paper "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy," presented to the Royal Society of Edinburgh, April 19, 1852, Professor Thomson arrives at the conclusions, that "there is at present in the material world a universal tendency to the dissipation of mechanical energy;" and that "within a finite period of time past the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present

*The whole amount of solar energy incessantly expended on our earth may be estimated at the amount of 208 billion 498,027 million "horse-power"-a horse-power being equal to 550 foot-pounds per second. But as was shown by the illustrious Dr. J. R. Mayer in his Beiträge zur Dynamik des Himmels, (Heilbronn, 1848,) the amount of solar heat intercepted by the earth is to the whole amount radiated into space "as 1 is to 2,300 millions." (On Celestial Dynamics. Translated by Dr. H. Debus, L. E. D Phil. Mag., April, 1863, Vol. XXV, p. 245.)