A body of apparent weight F, falls uniformly through a column of inviscid or frictionless fluid, contained in a vertical cylindrical or prismatic vessel, open at the top. Then the weight of the body (F) will be carried as additional pressure on the base of the vessel during the whole time of the descent; and if the vessel be tall and narrow the additional pressure will be approximately uniform and equivalent to an additional "head"; if it be wide, so that the walls are remote from the body, then the distribution on the pressure area will not be uniform, but will be greatest at the point vertically beneath the body, and less at points more remote. If the fluid possess viscosity, the whole of the force F may not reach the base of the vessel, but will in part be borne by its walls, but the total force carried by walls and base will in any case be equal to F.
In the above illustration there is nothing that is strikingly unfamiliar. If we suppose the vessel to be an ordinary jar of liquid, placed in the scale pan of a balance, there is a certain obviousness in the fact stated; the weight of the whole will be just the same whether the weight rests inert at the base of the jar, or whether it be falling uniformly through the fluid. When, however, the principle is applied to bodies aerodynamically supported in the free atmosphere the matter is not so self-evident; here, for example, we find that the weight of a parachutist is borne by the earth's surface almost from the moment he leaves the car, and his presence overhead, or the presence of a passing flight of birds, could be detected barometrically if we possessed an instrument of sufficient delicacy.
§ 7. Transmission of Force.—Comparison of Fluid and Solid.—We know that we may look upon a solid in stress as communicating momentum, since it transmits force, but a distinction must be drawn. When the flow of momentum is equal and opposite, as in the case of a solid in stress, there is no displacement of matter, and it is only when there is a displacement of matter that the Newtonian method can be applied. The case of a gas
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