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C. GENERAL PHYSICS.

THE RELATION BETWEEN GRAVITATION

ND ENERGY.

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In a paper on the relation of the law of gravitation to the principle of the conservation of energy, Rev. George P. Young, of Toronto, proposes to show that if this principle be accepted, it must follow that the force of gravitation, which, at ordinary sensible distances, is one of attraction, must at a certain limit necessarily undergo a transformation into a force of repulsion. He concludes, moreover, that there is a higher law than that of conservation of energy, which law is expressed by him in certain mathematical formula, from which he deduces both the law of gravitation at ordinary limits, and the law of repulsion within certain limits.-Canadian Journal of Science, XIV., 589.

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THOMSON'S DEAD-BEAT ARRANGEMENT FOR CHEMICAL

BALANCES. In giving an account of Sir William Thomson's arrangement for lessening the vibrations of delicate chemical balances, Professor Tait states that this method consists in suspending from the beam two very light closed cylinders, which fit closely, but without touching, into two fixed cylinders open at the top. This apparatus, when applied to a long and massive beam which ordinarily vibrated for some minutes when disturbed, brought it to rest in three half vibrations. There is therefore practically no limit to the length and consequently to the sensibility that may be given to the balance beam.

An instructive aero-dynamical phenomenon is noted by him in this connection, viz., the closed cylinder, when exactly balanced inside of the open cylinder, can be made to ascend briskly by a current of air

, even when the latter blows vertically downward upon the centre of the upper end of the cylinder.- Proceed. of the Roy. Soc. of Edinburgh, VIII., 491.

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SIMPLE DYNAMOMETER. Dr. Hamilton, of the Long Island College Hospital, mentions that, having a necessity for a dynamometer for therapeutical

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purposes, he devised the following construction: A glass tube dips into a bulb of pure India rubber. The interior of the bulb is filled with colored water, which, by pressure upon the bulb, is forced up in the tube, the upper part of which is filled with air, which is thereby compressed. Attached to the tube is a scale whose divisions represent pounds. As fifteen pounds' pressure to the square inch is required to compress the given body of air to one half its volume, of course that amount of force brought to bear upon the bulb will press the column of water half-way up the scale. This apparatus, besides the advantage of simplicity, cheapness, and great accuracy, has the further convenience that the bulb receiving the pressure is of a convenient shape, and can be used by persons of large or small hands. Again, the bulb is adapted to receive pressure exerted by all the flexors of the hands, and, finally, the action of the muscles is the same at different times, the same group of muscles being always brought into play, so that correct comparative tests may be made from day to dayPsychological and Medico-Legal Journ., April, 1875, 256.

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ON THE ELASTICITY OF ROCK-SALT.
Some physical experiments have been made by Pro-
fessor Groth, of Strasburg, upon the elasticity of rock-salt.
They involved, first, the preparation of rods of crystalline salt

, some three inches in length and one fiftieth of an inch in thickness; these were made perpendicular to the plane of the cube, and also to the dodecahedron. For the determination of the coefficient of elasticity the rod of salt was fastened to a much longer rod of brass, and then set in vibration in the usual way; sand strewed on the vibrating rods determined the position of the nodes, and these in connection with the thickness gave the elasticity of the salt relatively to the brass. By comparing two rods, made as described, the effect of the intervening brass rod was eliminated, and the coefficient of elasticity for the two determined. The ratio obtained was 1 to 11 (nearly), expressing the relation between the elasticity normal to the dodecahedral planes with that normal to the cubic planes. This result is important, as showing that the elasticity in an isometric mineral, like rocksalt, is not throughout uniform, but is dependent upon the direction.

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“FLOW 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.

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 of their members, whatever the material of wbich 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 completely 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, how. ever, 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,

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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 care-
fully 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 care-
fully passed into the glass, it will traverse the acetate solu-
tion 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 solu-
tion 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 disturb-
ing 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 ex-
periment anticipated quite unexpectedly.

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FRICTIONAL RESISTANCE OF WATER TO MOTION OF VESSELS.

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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

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