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excess, which is due to one section of the population (name-
THE DISTRIBUTION OF THUNDER-STORMS, Hildebrand has investigated the distribution of thunderstorms in Sweden for the year 1871, basing his study upon the observations of about two hundred and fifty stations. The number of days on which thunder was observed varied between one, two, and three in March, April, October, and November, to twenty-seven in July. Thunder-storms are found to be less frequent, not only in the cold months, but also as we proceed northwest. With regard to the daily distribution of these storms, he shows that the greatest number bave occurred between 4 and 6 P.M.; the least have occurred between 10 P.M. and 6 A.M. The greater number of thunder-storms come from the northwest; whence also come the clouds and wind. Le Verrier has long since shown that in France the thunder-storms occur on the advancing side of the general atmospheric disturbances, and that they follow the movements of the cyclones; but that, on the other hand, many thunder-storms are purely local. Mohn las likewise studied the subject in Norway, and comes to conclusions quite similar to those of Le Verrier. Mohn has furthermore shown that the origin of the thunder-storm is to be found in the ascent of warm, moist currents of air. These latter, however, also develop in connection with volcanic eruptions accompanied by whirlwinds, and also when the lowest strata of air are overheated in the hottest hours of the day. He therefore classifies the thunder-storms as whirlwind thunderstorms and heat thunder-storms. The former originate on the advancing side of cyclonic storms, and follow their movements over entire countries. The latter class originate on the hot summer afternoons, and are to be considered as purely local phenomena. If, however, we combine the observations of Mchn and Le Verrier with those of Breitenlohner, it will appear difficult to make so sharp a distinction as Mohn has attempted, since both causes are acting at the same time; so that extended and regularly advancing thunder
storms co-exist with merely local ones. The latter
indeed, generally sporadic, and especially frequent in favorable localities, such as mountains and forests. A passing cyclonic storm is always favorable to the formation of local thunderstorms. The peculiarly favorable conditions that prevail at certain localities are shown in a very interesting way in the work of Prettner on the climate of Carinthia. – VierteljahresRevue der Naturwissenschaften, II., II., 190.
A NEW BAROMETER OF LARGE SCALE. The great desirability of being able to observe the slightest changes in atmospheric pressure has led to the production of many more or less unsatisfactory barometers, of which in general it may be remarked that, although they do really afford us a highly magnified scale of movement, yet the moving parts are themselves so weighty that the sluggish behavior of the instrument entirely neutralizes the advantage which was sought, so that the slight momentary changes in atmospheric pressure still pass by unperceived. To meet these difficulties, Mr. Hirn, one of the most eminent French philosophers, has described an instrument which he calls the Megabarometer: his apparatus consists of three vertical glass tubes, closed at their ends, and connecting with a horizontal tube by means of iron sockets. The middle tube, filled with mercury up to half an inch of its top, is a true barometer. Its neighboring tube on the one side has about four millimeters' internal diameter, while the other tube has one millimeter diameter, but is soldered at its top to a closed bulb of about four centimeters' internal diameter. The lower half of this bulb is filled with mercury, the upper half with alcohol. The first of the three tubes thus constitutes a barometer composed of two liquids, and the variation of level in the two open tubes on the right and the left hand is very nearly in an inverse ratio to the densities of the liquids; so that a change of one inch in the height of the mercury brings about a change of seventeen inches in the alcohol tube.- Nouvelles Meteorologiques, p. 34.
MIRAGE. In some remarks on the phenomena of mirage, Professor Everett states that when a ray of light is passed through a portion of air which is not equally dense on all sides of the ray, it is deflected toward the side on which the density is greatest, the sharpness of the curvature being proportional to the rates at which the density varies. If the air is stratified horizontally, it follows that a ray traveling nearly horizontally will be bent the most, and it is by such rays that we see the images which constitute mirage. In the average state of the atmosphere the curvature of horizontal rays is about one fifth or one sixth of the curvature of the earth's surface, being greater in cold than in warm weather, and greater with high than with low barometer. The curvature, however, depends principally upon the rate at which the temperature changes with the height. The average rate is one degree for 300 feet. If the rate were one degree for fifty-three feet, the horizontal rays of light would be straight lines. A more rapid rate than this will render the air above denser than that below, and cause rays to bend up instead of down. This condition of affairs may exist for a time, although it is a condition of unstable equilibrium, and must eventually be broken up by the inflow from the surrounding regions of cold air. An increase of temperature upward at the rate of about one degree in sixteen feet will make the curvature of horizontal rays equal to that of the earth, so that they may encircle the globe. Any such downward bending of rays of light increases the range of our vision, enabling us to see around the horizon, which otherwise limits the view, thus bringing distant objects in sight, and rendering nearer objects more distinctly visible, but without in any way inverting them.-12 A, XI., 151.
THE NEW SELF-RECORDING BAROMETER, A self-recording barometer has recently been made by Mr. Redier, which seems to have many excellent points, and to be enthusiastically received in France. In a communication to the Meteorological Society of Paris, he states that his instrument consists of an ordinary syphon barometer carrying a very light ivory float, upon which is fixed a vertical steel wire terminating in a point. A horizontal needle rests upon this point, its other extremity is in connection with a double series of clock-work, the wheels of which move either forward
or backward, according as the ivory float rises or falls. The movements of the clock-work are followed by a pencil which draws a curve upon a revolving cylinder. In a very similar instrument devised by Professor G. W. Hough, of Albany, and highly prized by American meteorologists, the connection between the horizontal lever and the wheel-work is an electric one, and subject therefore to all the uncertainties of the electrical batteries and connections. In Redier's barometer no electricity is employed, the entire apparatus depending only upon gravity and atmospheric pressure, and its working is evidently perfectly regular and reliable. An aneroid barometer may be made to record its indications in the same manner as the mercurial, and such instruments have, we understand, already been constructed under Mr. Redier's directions. Mr. Silberman has suggested a method by which a similar automatic system of registration could be applied to the indications of the magnetic needle.--Nouvelles Meteorologiques, 1875, p. 16.
CURRENTS OF AIR WITHIN CYCLONES AND WATERSPOUTS.
In a memoir on cyclones and waterspouts, Mouchez publishes some observations made by him while upon
ocean, and which, if correct, are quite important. According to him, at or near the surface of the ground the movement of air in the cyclone is always from below upward, while in whirlwinds the movement is, on the contrary, from above downward. In the former case the winds are winds of aspiration; in the latter case the wind descends from the cloud in the form of a bag or tube, which terminates in a point. He believes that waterspouts have no relation whatever to cyclones, having an opposite appearance and cause. In this opinion Renou also concurs.-- Nouvelles Meteorologiques, 1874.
THE PROGRESSIVE MOVEMENT OF AREAS OF COLD AIR. Dove has attempted to deduce, from the five-day means of temperature for European stations, some general views as to the progress of days of remarkable cold, and finds that in the months of January and February of the years 1855, 1856, 1870, and 1871, numerous cases occurred to show, almost uniformly, that the “cold terms move westward over Europe;
a result that may, perhaps, be considered as entirely in agree ment with the deductions of Bachan, based upon the barometric charts prepared by bin.-Jonatsbericht Berlin Acad., Feb., 1974, 113.
A VERY DELICATE BAROMETER. An ingenious device has been constructed by Jendelef, which shows the slightest variations of pressure by means of a small C-shaped tube containing petroleum oil. One end of this tube is closed, and contains a certain volume of dry air maintained at a constant temperature, while the other end is open to the air. The instrument being accurately ad. justed by means of a mercurial plunger connected with the bottom of the U-shaped tube, so that the petroleum is exactly on a level in the two branches of the tube, it is found to be so extremely sensitive that the slightest variation of atmospheric pressure is shown by the alteration of the level, and the amount of this alteration can be measured with the greatest precision.-12 A, XI., 55.
TUE PENETRATION OF COLD INTO THE EARTH. From observations on the temperature of the surface of the earth which have been made regularly at the Botanical Gardens at Paris, by the Messrs. Becquerel, by means of the electric thermometer invented by them, some interesting conclusions have been deduced. Their observations have been extended to the depth of one hundred and seventeen feet; but most attention has been paid to that portion of the earth nearer the surface. With reference to the penetration of cold into soils of similar character, but one of which is bare and the other covered with grass, while both are cove ered with snow, the observations have shown that when the temperature of the air sinks from zero to -12° Centigrade, the temperature of the earth at the depth of twenty inches never sank to zero under the grass-covered earth, while it sank at the same depth to -5° under the bare surface. From these observations Becquerel makes the practical suggestion that if in a sandy soil we desire to cultivate plants whose roots suffer from frost, we must cover the soil with grass-sod; and, for similar reasons, if we desire to preserve vegetables or other products during the winter under the soil, and secure