RESISTANCE is a power by which motion, or a tendency to motion, in any body is impeded or prevented. When a weight or pressure acts upon a beam or bar in any direction, the tenacity by which the particles of such material oppose that action constitutes a — — — — resistance of one kind. [MArratAtis, S7IUNGT11 or.] Again, when a body is made to more on another, the inequalities of the surfaces of both create a resistance of a different kind. [Faissrlosa] When a body moved in a fluid. the inertia of the fluid particles displaced by it produces a third kind of resiatance.
This last branch of the subject of resistances has already been In part considered under HYDRODYNAMIC/S. in that article there is given a general expression for the measure of the resistance made by a fluid against a plane surface which is either perpendicular or Inclined to the direction of the motion, together with a few results of experi ments on the resistances experienced by bodies of various forms and lengths in moving through water. The relations between spaces and times In the vertical ascent and descent of bodies when acted on by gravity and resisted by a fluid, are given in the article P/IOJEC Tzt.r_s, Tncosv or ; and, for the pressure spinet a cannon-ball moving in air, see the article GUNNERY.
III investigating the resistances of fluids against bodies moving In them. it is customary in elementary writings, for the sake of simpli city, to consider the particles of fluid as unconnected with each other by contact or by any law of attraction, so that, when struck, their re actions may be considered as taking place perpendicularly to the strik ing surface of the moving body, whatever be the poeition of this surface with respect to the direction of the body's motion, and after the impact their action is supposed to cease. Such are called discon tinuous fluids, and in these the motion produced in the particles by the collision is the measure of the resistance. Newton shows (' Principle, lib, ii., prop. 35) what would be the resistance experienced by a cylinder moving in the direction of its axis in a discontinuous fluid ; the cylinder and particles of fluid being elastic, so that the latter on being struck are reflected back with a velocity double the velocity of the cylinder ; and lie explains that, if the particles of fluid are not reflected, but are moved forward by the cylinder with a velocity equal to its own, the resistance is but half Cho former. But
this hypothesis is far from being conformable to the constitution of fluid bodies in nature, the particles of these being connected by mutual actions. The elastic fluids, as air, at any place in the atmosphere are always iu a state of compression from the weight of the column vertically above that place ; and the particles of non elastic fluids, as water, exert in every direction Treasures which depend upon the distances of the particles below the surfaces of the fluid in the vessel, river, or ocean. In passing through a fluid of this kind (called a continuous fluid) a body strikes only the fluid particles which are nearest to it ; these strike those beyond, and so on; and Newton proves (lib. ii., prop. 35, echo].) that in this case the resistance to a cylinder is only half the last-mentioned resistance, or one-fourth of the first.
In all these resistances however it is supposed that the particles on being struck are repelled perpendicularly to the front of the moving body ; but, in fact, the particles of the fluid are in part repelled from the front in oblique directions, and, on account of the compressed state of the surrounding fluid, these particles not being able immedi ately to escape laterally, there is produced in front more or less con densation, and consequently an increase of resistance. The pressure of the fluid against the sides of the moving body creates also a resist ance from friction ; and when the velocity is very great, the fluid not falling towards the binder part of the body so fast as the latter moves, the pressure there which would serve to counterbalance the resistance in front, is in part or wholly removed. On these accounts it is that military projectilea are subject to such vast retarding forces. It is computed that a 24-pounder ball experiences a resistance equal to 80t) Ha. when its velocity is equal to 2000 feet per second. Like effects take place in the movements of boats and ships ; when the velocity is great, the water accumulates in front, and flowing off from thence obliquely, it carries away some from the sides, and, causing the surface of that which is near the stern to be rather lower than the general level, it there produces a diminution of pressure, while there is an caeca+ in front on account of the accumulation.