land, implying, of course, the establishment of a permanent hydrographic commission; second, that the most stringent laws be enacted to prevent the willful unnecessary destruction of forests; and, third, that every thing be done that is possible to encourage the planting and cultivation of forests; and, finally, that the streams and rivers, as they at present exist, be connected into a system by means of which the height of the water may be regulated, and the traffic therein. facilitated as much as possible.-Zeitschrift des Ingen.- und Architekten-Vereins, p. 137.

MAGNETIC PARTICLES IN ATMOSPHERIC DUST.

The atmospheric dust examined by Tissandier was col lected in several different ways, and in every case was found to contain minute magnetic ferruginous particles. Part of the dust was collected upon sheets of paper exposed to the air for many days; in another series of experiments air was passed through pure water, and the latter evaporated over sulphuric acid in vacuo. Other samples were collected from rain and snow water, in one instance, particularly, from snow gathered upon Mont Blanc, 2712 meters above sea-level. The ferruginous particles occurring in these various specimens of dust were extracted by means of a magnet and subjected to microscopic examination. In diameter they rarely exceed Tof a millimeter, and appear to consist of magnetic oxide, resulting from the combustion of iron. Tissandier believes them to be of cosmical, not terrestrial origin, and regards them as the debris of meteoric masses.-6 D, 1875, Oct. 4.

ON THE HEIGHT OF THE AURORA BOREALIS.

In discussing observations made by himself upon the aurora during the Swedish expedition of 1868 to the North Pole, Professor Lemström, of Helsingfors, states that although Loomis, and even Bravais, believed that observations which give a very low height to the aurora are erroneous, and the result of some illusion, yet he can not agree with them; and he offers in support of his opinion, among other things, the phenomena observed on the 18th of October, 1868, at the entrance of the Norwegian Archipelago, when the whole horizon was covered with rays which were soon united around the magnetic pole, forming a regular crown.

All the phenomena that he has obsérved and described in regard to the illuminated edges of clouds show very plainly that in these cases the polar light was produced in the region of the clouds, and even lower. We know by numerous observations that the number of thunder and lightning storms diminishes considerably as we approach the polar regions, so that they no longer occur in the latitude of 70°. Must we then conclude that in these regions the clouds are completely deprived of electricity? Certainly not; but only that the electrical discharges are made in some other way. In these high latitudes, electricity is discharged not only by clouds, but also directly by damp air, as takes place in the winter in the temperate zones. A great many direct observations prove the existence of slow discharges of this nature; and a very remarkable confirmation is given by Angström, who on one occasion proved the presence in the spectrum of the yellow or auroral ray over almost the entire sky. - Smithsonian Rep., 1874, 232.

RELATION OF THE PHASES OF THE MOON TO ATMOSPHERIC

PRESSURE.

In a memoir by Ludicke, he shows that the atmospheric pressure diminishes with the waxing and increases with the waning moon. The pressure is less at the perigee than at the apogee, and in general the effect of the moon upon the atmosphere is the inverse of that which it produces upon the ocean. The observations on which his results are based extend over eight years; but the actual effect of the moon upon the barometric pressure, although decided, is yet exceedingly small.

IRON IN ATMOSPHERIC DUST.

As the result of an examination of atmospheric dust, the mineral residuum from the melting of snow, etc., with special reference to iron in the atmosphere, Mr. Young remarks that this metal, in appreciable quantity, occurs in the dust accumulated in old buildings during a long period of time, and is usually of a globular form, showing that it has been formed at a high temperature. The iron as found in melted snow is much more irregular in shape, and is more abundant in proportion when the snow is collected at the lower levels. Mr.

Young has not been able to establish the occurrence of iodine in snow-water, as claimed by other experimenters.—6 B, 1876, July 17, 242.

SCHOTT'S TABLES OF ATMOSPHERIC TEMPERATURE.

An important work has recently been issued by the Smithsonian Institution, entitled "Tables of Atmospheric Temper ature in the United States," by C. A. Schott. The special result reached by Mr. Schott, in reference to secular variations of temperature, are of such general interest and impor tance that we quote some of his conclusions: "The character of the secular variation in the mean annual temperature is that of a series of irregular waves representing a succession of warmer and colder periods. These undulations, when computed for a number of stations exposed to similar climatological conditions, are seen to have approached to parallelism over large tracts of country. There is, however, nothing to countenance the idea of any permanent change in climate having taken place, or being about to take place. During the last ninety years the thermometric records of mean temperatures show no indication whatever of a sudden rise or fall." A similar conclusion has also been reached by Mr. Schott in reference to the rainfall. "If, now, we group together stations properly located, the undulations become well marked, the interval between successive maxima or minima being about twenty-two years on the Atlantic coast and seven years in the Mississippi Valley. Even these undulations, however, are not sufficiently regular to serve as a basis of prediction. A comparison of the temperature with the frequency of the solar spots show, so far as these records go, no traces of any direct connection between these phenomena. On comparing the temperatures and the rainfalls, there seems some ground for concluding that years of high mean temperature have also a large rainfall. And, again, for years of low temperature the winds appear to be northerly, and for high temperature southerly. The connection between the temperature, the rain, and the winds must, however, be ultimately considered as due to variations in solar radiation."

C. GENERAL PHYSICS.

THE RELATION BETWEEN GRAVITATION AND ENERGY.

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 formuiæ, 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.

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

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 day.Psychological and Medico-Legal Journ., April, 1875, 256.

ON THE ELASTICITY OF ROCK-SALT.

Some physical experiments have been made by Professor 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 14 (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.