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crease in the flood waters in cultivated lands, has lately been the subject of a report by the Vienna Academy of Science, from which it would appear that the diminution experienced in the course of the last few centuries in the average height of the water (especially the low waters) of the Rhine, the Elbe, and other rivers of Europe, is to be attributed to a diminntion in the annual quantity of water available for the rivers, and taking account of the amount of evaporation, there still results a diminution in the amount of water supplied by springs and rainfalls. It appears to the commission probable that the influence of forests is perceptible on the annual rainfall, and especially on the distribution of the rain throughout the year, although direct observations do not yet suffice to determine the magnitude of this influence. Among the causes tending to this diminution of the water in the rivers, the commission enumerate, first, the extinction of the forests, which exerted a beneficial influence in maintaining and elevating humidity, in diminishing the extremes of temperature, and diminishing the evaporation, and in assisting to a more equable drainage of the precipitation. A second cause is found in the drying up of the lakes, ditches, and morasses, which also would have exerted an influence similar to that of the forests. Third, in the cultivation of extended areas of land, which cultivation demands the consumption of considerable quantities of water. Fourth, in the increase of population, although the effect of such increase can directly account for only a slight percentage of the entire diminution. Finally, the commission consider the suggestion of Salmann as worthy of consideration, according to whom water is being employed in the interior of the earth in the formation of minerals which contain that liquid in chemical combination. In view of these considerations, the commission recommend that the Austrian government institute such observations as will lead to a further knowledge on this subject, and especially take such steps as will prevent the further diminution of water from becoming a calamity to future generations. Efforts will also be made to collect more complete data from the inhabited portions of the world; and especially is the hope expressed that the Viceroy of Egypt may cause tables and graphic representations to be made of the observations on the height of the water at the Nilometer at Cairo, where
such observations have been regularly made for the past 3000 years.
The actual diminution in the annual average height of the water, as deduced by Wex, is, for the different parts of the Rhine, from 6 to 60 inches during fifty years; for the Elbe, at Magdeburg, 17 inches; for the Oder, 17 inches; and for the Donau from 18 to 55 inches. — Sitzungsbericht der Akademie der Wissensch., Vienna, LXIX., April 23, 1874.
PHOTOGRAPHING THE WAVES. The various mathematical theories that have been, thus far, elaborated with reference to the movements of vessels upon the waves are only approximate. It is only by experience that we are able to determine to what degree of exactness these theories have arrived, and in order to verify them it is necessary to register the successive inclinations that a wave gives to a vessel. The photograph allows us to obtain the law of these inclinations. Let us suppose that a photographic apparatus, having its axis perpendicular to the diametral plane, be directed toward some point in the horizon. We should obtain on the sensitive plate an image of the sea and of the heavens, separated by a horizontal line, which would be the image of the horizon. Let us mark upon the plate the position of this image when the axis of the apparatus is upright. If it is then inclined by an angle, i, about a horizontal axis parallel to the diametral plane, the image of the horizon will remain parallel to the primitive line, but be displaced by a quantity equal to f, tang. i ; f being the focal distance of the objective. Let us suppose now that we place before the sensitive plate a fixed shutter pierced with a vertical slip; the image will be intercepted, except in that part of the plate situated behind the slip. We shall thus
the plate a broad band of two different tints corresponding to the sky and the sea, divided by a segment of the horizontal line. Consequently, if we take an instantaneous photograph at the moment when the ship is inclined at the angle i, we shall have a segment of the horizon line, and the distance of this line from the primitive horizontal line will give the angle i. In order to realize these conditions, it suffices to make the sensitive plate move horizontally with a uniform movement. If, during this movement, the
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vessel remains vertical, the segment of the image of the horizontal line will continue at a constant height upon the plate. Consequently it would trace upon the plate a horizontal line. Let us suppose now that the ship rolls; at each instant there would be a certain inclination, i, and at this moment a certain zone of the plate will be found behind the opening of the shutter. The image of the horizon will traverse this window at some point of this zone, and it will fall upon the plate at a distance from the reference line equal to f, tang. i. Experiments have been authorized according to this method, which is the invention of Huet, at Brest, and the photograph proofs show the good results that were obtained.
BAROMETRIC OBSERVATIONS ON THE OCEAN. Dr. Buys Ballot communicates in a preliminary way the results of the great unpublished work undertaken by the Meteorological Institute of the Netherlands, and which consists in the derivation from over three hundred thousand observations on board vessels of the average barometric pressure for each month, and for every five degrees square throughout the navigated portions of the North and South Atlantic Oceans. The average barometric pressure within ten degrees of the equator is 760.04 millimeters. The pressure within ten degrees of the parallel of 30° north is 765 millimeters; and within ten degrees of the parallel of 30° south it is 762.5. Beyond these latter parallels the pressure diminishes steadily toward the poles, and is, apparently, at the limits of Buys Ballot's tables, viz., about 50° of latitude, in the southern hemisphere, 750 millimeters, but in the northern hemisphere 760.- Oesterreich, Zeitschrift für Meteorologie, X., 159.
THE SMALL OSCILLATIONS OF THE BAROMETER. Hon. Ralph Abercrombie has examined the connection between the wind and the small oscillations of the barometer, He finds, for example, that with an open window looking south and the wind nearly south, in strong gusts the first movement of the barometer is always upward, and about one tenth of an inch, as if the effect of the wind on being resisted by the house was to compress the air in the room. In a corner house, one window open to the south and another to the west, the wind south, in strong gusts, with the west
window open, there were violent oscillations; but the first movement was always downward. Upon opening the south window as well as the west, the oscillations ceased. It is well known by medical men that some acute diseases are aggravated by strong winds; and the author has observed that this distress is associated with those small oscillations of the barometer. He suggests the following practical inethod of palliation. If windows can not be borne open, try, by closing or by otherwise arranging windows and doors, to diminish the distress. When, as in most cases, windows can not be opened, all doors and windows should be closely shut, as well as the vent of the chimney if there is no fire; and, if possible, the patient should be removed to a room on the lee side of the house.-12 A, XII., 78.
ON THE DISTRIBUTION OF BAROMETRIC PRESSURE IN EURO
PEAN RUSSIA. The well-known work of Buchan, published in 1866 by the Royal Society of Edinburgh, was the first that offered an extensive collection of barometric observations, and gave the first approximate idea of the correct isobaric lines throughout the whole world as drawn for every month in the year, and for the average of the entire year
itself, While Buchan in this work sought to obtain greater accuracy by using only the observations made between 1850 and 1860, he
however, frequently thrown into serious error by the uncertainties of the altitudes above sea-level of the stations in the interior of Europe and America. For the latter country annual barometric means have been of late published by the Army Signal-office; and we have now to record the appearance of an important work by Rikatcheff, on the distribution of barometric pressure in the interior of European Russia. The author states that, as regards Asiatic Russia, we have still too little material to justify the drawing of isobaric lines as Buchan has done. For European Russia he gives many, and in some respects important corrections to the figures employed by Buchan. Having personally inspected many of the Russian stations, Rikatcheff has been able to discriminate between the observations that are reliable and those which should be rejected; he has collected all the valuable barometric observations that have been made in
Russia, and has applied to those stations whose altitudes were correctly known by levelings the barometric reductions (as computed by Rühlmann's formula) necessary in order to reduce the observed pressures to the theoretical sealevel. The highest station to which he has applied this process is Tiflis, whose altitude is about 1500 feet, and at which altitude he states the reduction to sea-level already becomes quite uncertain. The longest series of observations employed by him is, for the Russian stations, that at Warsaw, embracing thirty-three years. Having reduced the annual pressures to sea-level for these stations, he has, by means of preliminary approximate isobars, determined the mean annual pressure at sea-level for the other stations whose altitudes were not correctly known, and has there computed the altitudes as dependent upon long series of good observations, of five interior stations, with a probable accuracy of less than forty feet, and has determined the altitude of eight other stations with an error of less than ten feet. These secondary stations thus became valuable in the formation of the monthly isobars, although, of course, they can not be used on the annual charts. The final charts thus completed by Rikatcheff present the same phenomena as those given by Buchan, but of not quite so extreme a nature. The change from a very high pressure in the interior of the continent during the winter to a low barometer in the summer time is
decided Jy marked (the explanation of this phenomenon given by Rikatcheft is too much at variance with mechanical laws to escape severe criticism). On the other hand, the annual chart shows a well-marked diminntion in the average barometric pressure as we proceed north ward. Its depression he attributes to the numerous cyclones that occur in this region, although he states very plainly that the average annual temperature, as well as the tension of aqueous vapor, should conspire to annul this effect. It is probable that at the time of composing his memoir he had not yet become acquainted with the writings of Ferrel, Thomson, Peslin, Colding, Everett, and others, by whom the depression of the barometer in the polar regions of the earth is very accurately explained as due to the rotation of the earth on its axis, combined with the general atmospheric currents. Ferrel has even explained, in a very perspicuous manner, the reason why this depression is