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During the years 1904-1905 the authors were engaged in a research on the resistance of certain kinds of iron and steel to reversals of direct stress.* I n these experiments the change from tension to compression was gradual, and followed an approximately simple harmonic law, special care being taken to avoid the subjection of the specimens to sudden shock, the effect of which, it was considered, would complicate the problem, and might conveniently form the subject of another research. Since in ordinary machine practice the stresses induced in the moving parts are, in general, due to a combination of shock and gradually applied load, as might happen in the case of the crank-pins and bearings of reciprocating engines, it appeared to the authors that the determination of the relative resistances to sudden shock of the same materials as used in the previous work on gradual reversals of stress would, combined with the previous results, be of considerable value to the designer. This determination appeared to be the more urgently required, since in the opinion of engineers of such wide

The paper deals with an experimental investigation of the similarity of motion in fluids, of widely different viscosities and densities, in motion relative to geometrically similar surfaces, the existence of which has been predicted from considerations of dynamical similarity by Stokes, Helmholtz, Osborne Reynolds, and Lord Rayleigh. The theory in its most general form may be expressed by the relation given by Lord Rayleigh R = ρ v 2 f ( v L/ v ), where R is the resistance per unit area of the surface, ρ the density of the fluid, v the velocity, L a linear dimension of the surface, v the kinematical coefficient of viscosity of the fluid, and the assumptions made in the derivation of the expression are that R depends solely on ρ , L, v , and v .

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