"FLOW 99 OF METALS.

Professor Thurston, who, nearly simultaneously with Commander L. A. Beardslee, ascertained that an increased power of resisting stress was developed in iron and steel by their subjection to a strain which produced distortion beyond the elastic limit, and gave them a set, has lately presented a paper to the American Society of Civil Engineers, in which he has given the results of extended researches as applied to other metals. He concludes that the simple extension or straining of any member of any metallic structure is not a cause of weakness except where it produces an actual reduction of section resisting rupture, or where it brings the line of stress into a new direction, in which it acts either with a larger component of force in the former direction of stress, or, as in the case of a reflexure of the metal, it takes the material at disadvantage strategetically after a new disposition of its particles has taken place.

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The conclusion seems also proper that the elevation of the elastic limit by strain can only occur in metals which are elastic, and are capable of being placed in a condition of reduced resisting power by internal stress, by artificial or external force. Finally, the conclusion has been arrived at that structures are not weakened by stresses exceeding the elastic power their members, whatever the material of which they are composed, and even when made of metals having no elasticity, and capable of yielding, like tin, by flow, unless such strains as are produced are productive of actual molecular disruption.Letter of R. H. Thurston.

THE PLASTICITY OF ICE.

Professor Dr. Pfaff has communicated to the Physical Society at Erlangen some experiments upon the plasticity of ice, his experiments being made with the object of obtaining some exact numerical data as to the pressure necessary to change the form of a mass of ice, since it is essentially interesting in the theory of glacial movements to determine the minimum pressure at which the ice is plastic. He finds that even the lightest pressure is sufficient to make one particle of ice slide from another, if it only acts constantly and at a temperature near the melting-point; that, in fact, near the melting-point ice behaves like wax. The results obtained by him com

pletely explain the fact that the movements of glaciers are more rapid in proportion as the temperature rises; and the theory of glacial movements seems to receive through his experiments a new support. - Sitzb. Physikal. - Medecin. Gesells., Erlangen, 1875, 72.

PLASTICO-DYNAMICS.

Within the last few years a new branch of mechanics has been developed by Saint-Venant, known as plastico-dynamics, which occupies itself with the movements in the interior of plastic solid bodies; researches on this subject were made by Tresca, and published in 1864 in a work on the flow of solid bodies, and in a work published nearly at the same time by Saint-Venant on the torsion of prisms of various shapes.

Subsequently, in 1870, Tresca published a work in which he endeavored to explain the phenomena under consideration by tracing out the movements of the molecules of the bodies in question as they moved more or less slowly under the influence of the pressures and resistances to which they were subjected. His principle of the conservation of volumes has been developed by Saint-Venant, who has shown, however, that it is not entirely adequate to explain the observed phenomena, or to give satisfactorily the actual values of the strain in the interior of solid bodies undergoing deformation in consequence of external pressures. Both Tresca and Saint-Venant agree that the fundamental principle of the new science amounts to this, that at every point in the interior of a plastic body which is being deformed the greatest tangential component of the pressure remains equal to a specific constant peculiar to each substance; and from this principle alone Saint-Venant has recently established certain differential equations of the plastic movements, an integration of which would go far toward a complete solution of the problem. Failing in an analytical solution, however, he suggests that experiments must be resorted to, and defines with some precision the nature of the desired observations.-6 B, LXXXI., 115.

AN EXPERIMENT IN INSTANTANEOUS CRYSTALLIZATION.

Péligot has recently described in La Nature the following interesting experiment: Dissolve one hundred and fifty parts,

by weight, of hyposulphite of soda in fifteen parts of boiling water, and gently pour the same into a tall glass jar so as to half fill it, keeping the solution warm by placing the glass in hot water. Then dissolve one hundred parts, by weight, of the acetate of soda in fifteen parts of hot water, and carefully pour it into the same glass, inclining the latter so that the second solution may gently flow upon the first, and form an overlying layer without mingling with it. When cool there will be two supersaturated solutions. If now a crystal of hyposulphite of soda be attached to a thread and carefully passed into the glass, it will traverse the acetate solution without disturbing it, but on reaching the hyposulphite solution will cause the latter to crystallize instantaneously in large rhomboidal prisms with oblique terminal faces. When the lower solution is completely crystallized, a crystal of the acetate of soda similarly lowered into the upper solution will cause it to crystallize in oblique rhombic prisms. The appearance of the two different kinds of crystals, and the method of their formation by the selection of the disturbing crystal, forms an interesting philosophical experiment. Though not mentioned in our source of information, it would be well to observe the precaution of covering the surface of the upper solution with a thin layer of oil, to protect it against disturbance and the falling in of dust-particles, by which the crystallization will readily be set up, and the foregoing experiment anticipated quite unexpectedly.

FRICTIONAL RESISTANCE OF WATER TO MOTION OF VESSELS.

In the report to the Lords Commissioners of the Admiralty on the experiments for the determination of the frictional resistance of water to moving surfaces under various conditions, it is stated that a number of surfaces were experimented with, the experiments being conducted by drawing known areas of the given body through the fluid. The results may be expressed generally by a formula based upon the assumption that the resistance offered to the particles of fluid is purely dynamic, dependent upon the weights of the particles and the velocities imparted to them. Consequently, for lengths of surface above fifty feet, the increase of the friction per square foot of every additional length is so small that it will make no very great difference in our es

timate of the total resistance of a surface three hundred feet long, such as the side of a ship, whether we assume such increase to continue at the same rate throughout the last two hundred and fifty feet of the surface, or to cease entirely af ter fifty feet.-Proc. Brit. School of Adv. Sci., 1874.

THE MOVEMENTS OF WAVES AND VESSELS AT SEA.

The study of the motions of vessels is prosecuted in the French navy with considerable diligence by means of the apparatus invented by Madamet, and described in a note in the Maritime and Colonial Review, May, 1876. This appa ratus consists essentially of a Foucault regulator causing two pencils to traverse a cylinder with perfect regularity; the whole apparatus being swung on gimbals in such a way that every movement of the vessel is registered, as regards direction, velocity, and time, on a sheet of paper. This apparatus has been used on board of a number of French frigates, such as L'Océan, La Minerve, and La Galissonniere, while lying in the port of Brest, in order to determine completely the movements of vessels oscillating in a calm sea. At present the apparatus is employed in connection with that devised by Risbee, in order to measure the resistance of a vessel to careening.-Revue Maritime et Coloniale, May, 1876, 481.

VIBRATION OF FLUID COLUMNS.

The fact that a column of air can be set into longitudinal vibration has suggested to Kundt the possibility of producing similar longitudinal vibrations in columns of water. To this end he sets the tube containing the water in vibration, and finds that the success of his experiment depends principally upon removing from the liquid all traces of gas, whether the latter be absorbed or are in the shape of minute bubbles. The gases absorbed by the liquid are partially driven out by the vibrations, and appear as disturbing bubbles. The so-called tone figures of Kundt can be shown in the vibrating columns of liquid as well as in those of air, and can be used to determine the velocity of sound in the liquid. It is found that the thickness of the glass tube has an influence on the velocity of sound, such that the thicker the tube the more rapid is the propagation of the sound wave.-19 C, VIII, 7.

ON THE THEORY OF THE FLOW OF WATER OR GAS.

The important work of Boussinesq, entitled "An Essay on the Theory of Currents of Water," which will be printed by the Academy of Sciences of Paris in the twenty-third volume of its foreign memoirs, has received some interesting additions from its author, among which is one that treats on the transpiration of gases, which interesting subject is known to us at present only through the observations of Grohan, Exner, and others, but whose results are explained by the mechanical theories of Boussinesq. Other additions are those that relate to the uniform regimen of flowing water, and especially, the third addition, that which serves to complete the theory of vertical waves, in which the author calculates the loss of dynamic energy that the waves experience as a consequence of the mutual friction of the layers of liquid as well as the friction against the containing walls. -6 B, LXXXI., 466.

HYDRAULIC INVESTIGATIONS IN INDIA.

A very extensive series of experiments by Captain Allen Cunningham on the flow of water in rivers and canals is published as an extra to the professional papers on Indian engineering. After noticing the results derived from Humphrey and Abbott's study of the Mississippi, D'Arcey and Bazeir's studies in France, and Redy's studies on the Paraguay, Uruguay, and La Plata, he states that the primary object in his own investigations has been to test the applicability of the results of the American and French experiments to large bodies. of water in regular channels.

Among the conclusions to which his own observations have led him are the unsteadiness in the motion of a large body of water, even in a uniform channel of great length; the velocity at any point varying considerably from one instant to the next, although the average velocity is sensibly constant. The curve representing the surface velocity of a current of water is strikingly regular, and is symmetrical with reference to the mid-channel, while its form depends upon the figure of the cross-section. In a very wide channel it has a flat curve. In a rectangular section in masonry it is approximately a quartic ellipse. The surface velocity near the mar