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FIGURES MADE BY LIGHTNING. Professor Tomlinson, in writing on the subject of the fig. ures or marks left on the bodies of men or animals killed by strokes of lightning, states that very instructive tree-like figures may be produced on sheets of ground glass by passing over them the contents of a Leyden-jar. These figures, like those on the human subject, are not derived from any tree whatever, but represent the path of the lightning itself. This subject has been studied by numerous authors; among others by Poey, who, in 1861, published a small volume, in which twenty-four illustrative cases are cited. He accounts for their formation as a photo-electric effect, in which the surface of the animal is the sensitive plate, the tree, etc., the object, and the lightning the force that impresses it. Among many remarkable cases that can be quoted was one that occurred at Zante, where the mast of a vessel was struck, and a sailor sleeping in a cot on the deck was killed. The number 44, in metal, was attached to the fixed rigging between the mast and the cot. On the left breast of the dead sailor was found the number 44 well formed, and perfectly identical with that on the rigging Light was thrown upon these cases by Mr. Varley, who, noticing some specks on the metallic ball of the positive pole of a Holtz electric machine, tried to wipe them off with a silk handkerchief, but in vain. He then examined the negative pole, and discovered a minute speck corresponding to the spots on the positive pole. It was evident that lines of force existed between the two poles, by means of which, as it were, telegraphic communication was made from one to the other; and in explanation of the marks made on the human subject, it is stated that a lightning burn on the skin is produced whenever the object struck is electrically positive to the metallic object, the discharge itself being a negative one.
CLIMATOLOGY OF FLORIDA. In an address on the climatology of Florida, recently delivered by Dr. Baldwin, the author gives some interesting statistics, whose value is indicated by the fact that they are based upon thirty-six years' meteorological observations, recorded by himself, at Jacksonville, together with numerous shorter records from the stations in other portions of the state. He states that the first frost has occurred in the fall in October four times in the course of these records, in November sixteen times, and in December seven times. There have been several years in which there have been no frosts in October. There have been three years in which none has occurred in November nor December. Of late frosts he says that there have been very few in April, and none after that. The latest on record is that on the 28th of April, 1858. There have been but four Aprils and but four Octobers in which frosts have been recorded. From these statements an idea can be formed of the average amounts of freezing weather in winter. Frosty days occur, on the average, abont five days in each of the months of December and January. As to clear days, he states that from November to March there is an average of twenty clear days per month; but for June, July, Angust, September, and October an average of from seventeen to nineteen days. Of rainy days there are in January six or seren, in February three or four, in March five or six, and in December five. “I judge,” Dr. Baldwin says, “ that, on the whole, the preponderance of clear over rainy and cloudy days speaks decidedly in favor of our climate as being characterized by a fair amount of pleasant weather. The excessively cloudy weather of January, 1875, is a marked exception to all former years since my residence in Florida, and has most probably resulted from some general disturbance of the atmosphere, which has produced such intense cold in the Northern States as will probably be remembered hereafter as one of those cold winters which at long intervals will visit a conntry, and which on many accounts may be coilsidered as a blessing in disguise.'”–Baldwin's Address, 1875.
C. GENERAL PHYSICS.
ATTRACTION AND REPULSION RESULTING FROM RADIATION.
Mr. Crooks has lately published his investigations into the phenomena known as attraction and repulsion resulting from radiation. The apparatus constructed by him appears to be more sensitive than the ordinary thermo-multiplier. He considers that the experiments show that the repulsion is not entirely due to the rays usually called heat, viz., to the extreme and ultra red rays of the spectrum. The theory advanced by Professor Reynolds to explain his observations is not acceptable to him, although, on the other hand, he has not yet prepared one of his own. According to Reynolds and Balfour Stewart, these experiments constitute a direct proof, and the only known direct proof of the truth of the kinetic theory of gases as developed by Clausius and Maxwell.
Mr. Crooks has adapted his results to the construction of a very important instrument, which he calls a radiometer. It consists of four arms suspended on a steel-pointed axle resting in a cup, so that the arms are capable of revolving horizontally. To the extremity of each arm is fastened a thin disk of pith, lampblacked on one side, and the whole is inclosed in a very perfect vacuum within a glass globe. Under the influence of light, or heat, the little arms revolve with considerable rapidity. Mr. Crooks states that the repulsion experienced by these disks when any radiation falls upon them is proportional to the length of the vibrations, and varies at every point of the spectrum. Professor Guthrie has remarked that Mr. Crooks's research had, in an almost unequaled degree, every element of greatness.
INCREASE OF RADIATION WITH TEMPERATURE. It is well known that as the temperature of a solid is gradually increased, the refrangibility of the emitted light increases likewise, and as the result we find red light emitted first; afterward the other colored rays gradually appear as the heat increases, until we reach the ultra-violet rays.
This correlation between refrangibility and temperature was first
experimentally proved by J. W. Draper, and it would be a lesult of great importance to determine the law of growth of refrangibility with temperature. If this could be achieved, a very convenient and accurate pyrometer could be made of the ordinary spectroscope. Au accurate investigation of this subject has been undertaken by a committee of the British Association, which has in a preliminary report presented some observations on the simple increase of radiation with temperature. On this subject Becquerel has a great number of observations, whence be infers that the differences between the logarithms of the luminous intensities are proportional to the differences of temperature, a law which he thinks would hold up to 1200 degrees Centigrade; but the law as thus expressed mathematically by no means represents the true rate of increase of the total luminous intensity, which is, indeed, very much slower than that required by Becquerel's law. Again, if the law of Dulong and Petit for the velocity of cooling be true, then the amount of heat radiated, as also the temperature, could be calculated; but on comparison with actual observations at high temperatures it is found that their law gives too rapid an increase for the total radiation. Assuming, however, these laws to be even approximately correct, we may calculate the hypothetical temperature corresponding to the brightness and total radiation from the sun, and deduce in one case for the solar temperature 13,000, and in the other case 11,000 degrees Centigrade. - Report of the British Association, 1873, 461.
THE DIFFUSION BETWEEN DRY AND MOIST AIR, An investigation that may have some interest in the future of meteorology has been conducted by Dufour, who has examined the question of the diffusion between dry and moist air traversing a porous disk. He finds that the activity of the diffusion does not depend directly, except possibly in a very slight degree, upon temperature. If we compare the observations made at different temperatures, we find that the activity of the diffusion also does not depend, with slight exception, upon the relative humidity; it depends principally upon the difference between the quantities of vapor, or the tensions of the vapor, on the opposite sides of the disk, and is in fact very nearly proportional to the difference of the ten
sions. If, then, we compare observations made at the same temperature, the activity depends upon the relative humidity of the two portions of air. The study of the influence of the diaphragm, its extent, etc., has enabled him to explain how a small porous vase can, by connecting its interior with a manometer, be utilized in studying the diffusion of vapor in the free atmosphere. Similarly, we may determine the tension of the vapor of water existing in the air, at any moment, by determining the difference of the pressures shown by the monometer in the interior, and by the barometer on the exterior. This suggests a new hygrometer, which, simple as it is, and easily applicable, may rapidly find favor among meteorologists so soon as Dufour has exactly developed the laws connecting the relations of pressure and the hygrometric state of the atmosphere.-Bibl. Univers. et Revue Suisse, 1874, 336.
ON THE LAWS OF APPARENT ADHESION. Stefan, who is well known as one of the most industrious investigators in all matters relating to molecular physics, has recently published in full a memoir relating to preliminary investigations on the subject of adhesion. The true phenomena of adhesion are easily confounded with what he calls apparent adhesion. That is to say, if two perfectly flat plates be brought quite near together, but not in actual contact, and an attempt be made to separate them, a slight force is required, which Stefan states, on further investigation, he is persuaded is simply the result of the resistance which the viscosity of the liquid or gas offers to its immediate inflow into the space between the two disks. The effect of this viscosity, of course, would disappear if the experiment could be performed in an absolute vacuum. On the other hand, the study of the experiments made by Stefan in the atmosphere at its ordinary pressure, and of such liquids as alcohol, water, solutions of salt, etc., has enabled him to determine the true co-efficient of viscosity or internal friction for these substances. The values he arrives at are, for water, 0.0108, for air, 0.00183; figures which agree exactly with those deduced by Maxwell and Meyer by entirely different methods of experiments. The actual separation by means of a slight force of the disks experimented with by Stefan is, he states, a dynamic, and not a static phenomenon. The time