Resisting Force, in Philosophy, denotes, in general, any power which acts in an opposite direction to another, so as to destroy or diminish its effect. See Mechanics, Hydrodynamics, and Pneumatics.

Of all the resistances of bodies to each other, there is undoubtedly none of greater importance than the resistance or re-action of fluids. It is here that we must look for a theory of naval architecture; for the impulse of the air, which is our moving power, must be modified so as to produce every motion we want by the form and disposition of our sails; and the resistance of the water, which is the force to be overcome, must also be modified to our purpose, in order that the ship may not drive like a log to leeward, but, on the contrary, may ply to windward; that she may answer her helm briskly, and be easy in all her motions on the surface of the ocean. The impulse of wind and water makes these elements ready and indefatigable servants in a thousand shapes for driving our machines, and we should lose much of their service did we remain ignorant of the laws of their action; they would sometimes become terrible masters, if we did not fall upon methods of eluding or softening their attacks.

We cannot read the accounts of the naval exertions of Phoenicia, Carthage, and of Rome, exertions which have hardly been surpassed by anything of modern date, without believing that the ancients possessed much practical and experimental knowledge of this subject. It was not, perhaps, possessed by them in a strict and systematic form, as it is now taught by our mathematicians; but the master-builders, in their dockyards, did undoubtedly exercise their genius in marking those circumstances of form and dimension which were, in fact, accompanied with the desirable properties of a ship, and thus frame to themselves maxims of naval architecture, in the same manner as we do now.

The ancients had not made any great progress in the physico-mathematical sciences, which consist chiefly in the application of analysis to the phenomena of nature; and in this branch, in particular, they could make none, because they had not the means of investigation. A knowledge of the motions and actions of fluids is accessible only to those who are familiarly acquainted with the fluxionary mathematics; and without this key there is no admittance. Even when possessed of this guide, our progress has been very slow, hesitating, and devious; and we have not yet been able to establish any set of doctrines which are susceptible of an easy and confident application to the arts of life. If we have advanced farther than the ancients, it is because we have come after them, and have profited by their labours, and even by their mistakes.

Sir Isaac Newton was the first who attempted to make the motions and actions of fluids the subject of mathematical discussion. He had invented the method of fluxions long before he engaged in his physical researches, and he proceeded in these sua mathe* facem preferente. Yet even with this guide he was often obliged to grope his way, and to try various by-paths, in the hope of obtaining a legitimate theory. Having exerted all his powers in establishing a theory of the lunar motions, he was obliged to rest contented with an approximation instead of a perfect solution of the problem which ascertains the motions of three bodies mutually acting on each other. This convinced him that it was, in vain to expect an accurate investigation of the motions and actions of fluids, where millions of unseen particles combine their influence. He therefore endeavoured to find some particular case of the problem which would admit of an accurate determination, and at the same time furnish circumstances of analogy or resemblance sufficiently numerous for giving the limits of those other cases of Fluids, that did not admit of this accurate investigation.

Newton figured to himself a hypothetical collection of matter possessing the characteristic property of fluidity, viz., the quaquaversum propagation of pressure, and the most perfect immobility of parts, and forming a physical whole or aggregate, whose parts were connected by mechanical forces determined both in degree and in direction, so that the determination of certain important circumstances of the motion of the parts might be rendered susceptible of precise investigation. And he concluded that the laws which he should discover in these motions must have a great analogy with the laws of the motions of real fluids; and from this hypothesis he deduced a series of propositions, which form the basis of almost all the theories of the impulse and resistance of fluids which have been offered to the public since his time.

It must be acknowledged that the results of this theory agree but ill with experiment, and that, in the way in which it has been prosecuted by subsequent mathematicians, it proceeds on principles or assumptions which are not only gratuitous, but even false. But, with all its imperfections, it still furnishes (as was expected by its illustrious author) many propositions of immense practical use, they being the limits to which the real phenomena of the impulse and resistance of fluids really approximate; so that when the law by which the phenomena deviate from the theory is once determined by a well-chosen series of experiments, this hypothetical theory becomes almost as valuable as a true one. It continues to be the groundwork of all our practical knowledge of the subject.

We shall therefore lay before our readers a very short view of the theory, and the manner of applying it; we shall then show its defects (all of which were pointed out by its great author), and recount some of the attempts which have been made to amend it; and, lastly, we shall give an account of the chief sets of experiments which have been made on this important subject, in the hope of establishing an empirical theory, which may be employed with confidence in the arts of life.

We know by experience that force must be applied to a body in order that it may move through a fluid, such as air or water; and that a body projected with any velocity is gradually retarded in its motion, and generally brought to rest. Analogy leads us to imagine that there is a force acting in the opposite direction, or opposing the motion, and that this force resides in or is exerted by the fluid; and the phenomena resemble those which accompany the known resistance of active beings, such as animals; therefore we give to this supposed force the metaphorical name of Resistance. We also know that a fluid in motion will hurry a solid body along with the stream, and that force is required to maintain it in its place. A similar analogy makes us suppose that the fluid exerts force, in the same manner as when an active being impels the body before him; therefore we call this the Impulsion of a Fluid. And as our knowledge of nature teaches us that the mutual actions of bodies are in every case equal and opposite, and that the observed change of motion is only the indication and measure of the changing force, the forces are the same, whether we call them impulsions or resistances, when the relative motions are the same, and therefore depend entirely on these relative motions. The force, therefore, which is necessary for keeping a body immovable in a stream of water flowing with a certain velocity, is the same with what Resistance is required for moving this body with this velocity through of Fluids, stagnant water.

A body in motion appears to be resisted by a stagnant fluid, because it is a law of nature that force must be employed in order to put any body in motion. Now the body cannot move forward without putting the contiguous fluid in motion, and force must be employed for producing this motion. In like manner, a quiescent body is impelled by a stream of fluid, because the motion of the contiguous fluid is diminished by this solid obstacle; the resistance, therefore, or impulse, no way differs from the ordinary communications of motion among solid bodies.

Sir Isaac Newton, therefore, begins his theory of the resistance and impulse of fluids, by selecting a case where, although he cannot pretend to ascertain the motions themselves which are produced in the particles of a contiguous fluid, he can tell precisely their mutual ratios. He supposes two systems of bodies such, that each body of the first is similar to a corresponding body of the second, and that each is to each in a constant ratio. He also supposes them to be similarly situated, that is, at the angles of similar figures, and that the homologous lines of these figures are in the same ratio with the diameters of the bodies. He farther supposes that they attract or repel each other in similar directions, and that the accelerating connecting forces are also proportional; that is, the forces in the one system are to the corresponding forces in the other system in a constant ratio, and that, in each system taken apart, the forces are as the squares of the velocities directly, and as the diameters of the corresponding bodies, or their distances, inversely.

This being the case, it follows, that if similar parts of the two systems are put into similar motions in any given instant, they will continue to move similarly, each correspondent body describing similar curves, with proportional velocities: For the bodies being similarly situated, the forces which act on a body in one system, arising from the combination of any number of adjoining particles, will have the same direction with the force acting on the corresponding body in the other system, arising from the combined action of the similar and similarly directed forces of the adjoining correspondent bodies of the other system; and these compound forces will have the same ratio with the simple forces which constitute them, and will be as the squares of the velocities directly, and as the distances, or any homologous lines, inversely; and therefore the chords of curvature, having the direction of the centripetal or centrifugal forces, and similarly inclined to the tangents of the curves described by the corresponding bodies, will have the same ratio with the distances of the particles. The curves described by the corresponding bodies will therefore be similar, the velocities will be proportional, and the bodies will be similarly situated at the end of the first moment, and exposed to the action of similar and similarly situated centripetal or centrifugal forces; and this will again produce similar motions during the next moment, and so on for ever. All this is evident to any person acquainted with the elementary doctrines of curvilinear motions, as delivered in the theory of physical astronomy.

From this fundamental proposition, it clearly follows, that if two similar bodies, having their homologous lines proportional to those of the two systems, be similarly projected among the bodies of those two systems with any velocities, they will produce similar motions in the two systems, and will themselves continue to move similarly, and therefore will, in every subsequent moment, suffer similar diminutions or retardations. If the initial velocities of projection be the same, but the densities of the two systems, that is, the quantities of matter contained in an equal bulk or extent, be different, it is evident that the quantities of motion produced in the two systems in the same time will be proportional to the densities; and if the densities are the same and uni-

form in each system, the quantities of motion produced will be as the squares of the velocities, because the motion communicated to each corresponding body will be proportional to the velocity communicated, that is, to the velocity of the impelling body; and the number of similarly situated particles which will be agitated will also be proportional to this velocity. Therefore the whole quantities of motion produced in the same moment of time will be proportional to the squares of the velocities. And, lastly, if the densities of the two systems are uniform or the same through the whole extent of the systems, the number of particles impelled by similar bodies will be as the surfaces of these bodies.

Now the diminutions of the motions of the projected bodies are (by Newton's third law of motion) equal to the motions produced in the systems; and these diminutions are the measures of what are called the resistances opposed to the motions of the projected bodies. Therefore, combining all these circumstances, we have the following proposition.

Prop. I. The resistances and (by the third law of motion) the impulsions of fluids on similar bodies, are proportional to the surfaces of the solid bodies, to the densities of the fluids, and to the squares of the velocities, jointly.

We must now observe, that when we suppose the particles of the fluid to be in mutual contact, we may either suppose them elastic or unelastic. The motion communicated to the collection of elastic particles must be double of what the same body, moving in the same manner, would communicate to the particles of an unelastic fluid. The impulse and resistance of elastic fluids must therefore be double of those of unelastic fluids. And thus the fundamental proposition of the impulse and resistance of fluids, taken in its proper meaning, is susceptible of a rigid demonstration, relative to the only distinct notion that we can form of the internal constitution of a fluid. We say taken in its proper meaning; namely, that the impulse or resistance of fluids is a pressure, opposed and measured by another pressure, such as a pound weight, the force of a spring, the pressure of the atmosphere, and the like. We apprehend that it would be very difficult to find any legitimate demonstration of this leading proposition different from this, which we have now borrowed from Sir Isaac Newton, Prop. 23, b. ii. Princip. We acknowledge that it is prolix, and even circuitous; but in all the attempts made by his commentators and their copyists to simplify it, we see great defects of logical argument, or assumption of principles which are not only gratuitous, but inadmissible.

Before we proceed farther, it will be proper to make a general remark, which will save a great deal of discussion, and since it is a matter of universal experience, that every action of a body on others is accompanied by an equal and contrary reaction; and since all that we can demonstrate concerning the resistance of bodies during their motions through fluids proceeds on this supposition (the resistance of the body being assumed as equal and opposite to the sum of motions communicated to the particles of the fluid, estimated in the direction of the body's motion), we are entitled to proceed in the contrary order, and to consider the impulsions which each of the particles of fluid exerts on the body at rest, as equal and opposite to the motion which the body would communicate to that particle if the fluid were at rest, and the body were moving with equal velocity in the opposite direction. And therefore the whole impulsion of the fluid must be conceived as the measure of the whole motion which the body would thus communicate to the fluid. It must therefore be also considered as the measure of the resistance which the body, moving with the same velocity, would sustain from the fluid. When, therefore, we