Example:-In latitude 21° on the mountain of Guanaxato, in Mexico, Baron von Humboldt observed as follows: what was the difference of level between the stations? Log B'=1.37401 B=0.00031 1.37432 Log B 1.47784 '=70·4 f' = 23.66 ARTICLE VI. SECOND DIVISION, SECTION 3. ON ATMOSPHERIC WAVES AND BY WILLIAM RADCLIFF BIRT, Esq. F.R.A.S. INTRODUCTION. PROFESSOR DOVE, of Berlin, has suggested that in the temperate zones the compensating currents of the atmosphere, necessary to preserve its equilibrium, may be arranged as parallel currents on the surface, and not superposed or existing perpendicularly over each other, as in or near the torrid zone, and as illustrated by Lieutenant Maury in his Physical Geography of the Sea.' The views of Professor Dove may be thus enunciated:-That in the parallels of Central Europe the N.E. current flowing towards the equator, to feed the ascending column of heated air, is not compensated by a current in the upper regions of the atmosphere flowing from the S.W., as in the border of the torrid zone, but there are also S.W. currents on each side the N.E., which, to the various countries over which they pass, appear as surface winds; in fact being disposed in alternate beds or layers S.W., N.E., as in figure 1. The Professor also suggests that these parallel and oppositely directed winds are shifting, i. e. they gradually change their position with a lateral motion in the direction of the large arrow cutting them transversely. In the course of the writer's researches on atmospheric waves he had an opportunity of testing the correctness of Professor Dove's suggestion, which he found to be in close accordance with the truth, for he not only ascertained the existence of the S.W. and N.E. compensating currents, but also that of another set of oppositely directed and compensating winds at right angles to them. These were N.W. and S.E., with a lateral motion towards the N.E. The writer also carefully discussed the barometric phenomena with relation to both these sets of currents, and arrived at conclusions which the reader will find in detail in the writer's third report presented to the British Association for the Advancement of Science (Reports, 1846, pp. 132 to 162), but which may be briefly enunciated as follows:-During the period which passed under the examination of the writer, he found the barometer generally rising with N.E. and N.W. winds, and that, as a maximum or highest reading of the mercury approached, the wind died away mostly to a calm. On the other hand, with S.W. and S.E. winds the barometer generally fell, the force of the wind proportionally increasing until the mercury passed its minimum or lowest reading. From these considerations it is clear that the phenomena may be thus illustrated:-Let the strata a a a' a', b' b' b b, represent two parallel aërial currents or winds a a a' a' from S.W. or S.E., and b' b' b b from N.E. or N.W., and conceive them both to advance from the N.W. in the first instance, and from the S.W. in the second, in the direction of the large arrow. Now conceive the barometer to commence rising just as the edge bb passes any line of country, and to continue rising until the edge bb arrives at that line, when the maximum or highest reading is attained. It will be remarked that this rise is coincident with a N.E. or N.W. wind. The wind now changes and the barometer begins to fall, and continues to fall until the edge a a coincides with the line of country on which bb first impinged. During this process we have all the phenomena of an atmospheric wave; in fact the combined phenomena consisting of the two oppositely directed winds, their decrease and increase of force, accompanied by a rising and falling barometer, constitute in the true acceptation of the term an atmospheric wave; for when the edge b b passes a line of country the barometer is at a minimum. This minimum has been termed the anterior trough: it is characterised by a strong wind, often amounting in force to that of a hurricane. During the period the stratum b' b' b b transits, the barometer rises, while the wind decreases in force: this rise has been called the anterior slope. When the conterminous edges of the strata a a' bb pass, a barometric maximum extends along the line of country formerly occupied by the anterior trough: this maximum has been designated the crest, and is marked by a calm state of the atmosphere, and accompanied by settled weather. During the transit of the stratum a' d' a a, the barometer falls, and the wind increases in force; this fall has been denominated the posterior slope. When the edge a a occupies the place of bb, the descent of the mercurial column is completed, another minimum extends in the direction of the former, and this minimum has been termed the posterior trough. Figure 2 illustrates the general phases of an atmospheric wave. The localities in which atmospheric waves have hitherto WIND WIND been studied have been confined to the northern and central parts of Europe-the west of Ireland, Alten in the north of Europe, Lougan near the sea of Azov, and Geneva being the outlying points of the area over which they have been known to traverse. A most interesting instance of an atmospheric wave traversing Europe from the west of Ireland to the Black Sea is recorded in the French serial work, Cosmos;' and, in the Mercantile Marine Magazine' for May, 1856, we find a notice of the occurrence of an atmospheric wave in the Indian Ocean between 40° and 50° south latitude and 44° and 56° east longitude. It will be remarked that the greatest portion of the European area is inland, but there is one important feature which the study of the barometer has brought to light, and which is by no means devoid of significance, viz. that its oscillations are much greater in the neighbourhood of water: thus it appears that the junction lines of land and water form by far the most important portions of the globe in which to study atmospheric waves, for in the great systems of European undulations it is well known that these oscillations increase especially towards the north-west, and the late Professor Daniel has shown from the Man WIND WIND |