ART. VI. On the bearing of the Barometrical and Hygrometrical Observations at Hobarton and the Cape of Good Hope on the general theory of the Variations of Atmospherical Phenomena; by Professor Dove of Berlin.*

I HAD hoped to have prefaced this volume with a discussion of the meteorological observations made hourly at Hobarton from January, 1841, to September, 1848 (of which the abstracts were published in 1850 in the first volume of the Hobarton Observations), from the pen of Professor Dove, who had kindly undertaken, at the magnetical and meteorological conference at Cambridge in 1845, to participate to that extent in the reduction and application to theoretical conclusions, of the results of the Observations at the British Colonial Observatories; but M. Dove's appointment, on the death of Professor Mahlmann in November, 1848, to the charge of the meteorological observatories in the Prussian states has materially abridged the time at that gentleman's disposal, and he has found himself unable to complete the discussion he had undertaken for the present volume without occasioning an inconvenient delay in its publication; the discussion will therefore be prefixed to the fourth volume; but in the mean time Professor Dove has kindly furnished for this volume the subjoined remarks (written in German) upon the bearing which the barometrical and hygrometrical observations, at the Colonial Observatories at Hobarton and the Cape of Good Hope, have had on the general theory which professes to explain the physical causes of the variations which we observe in the atmospherical phenomena of the globe. The testimony borne by so eminent a meteorologist to the importance and value of this portion of the observations made at the British Colonial Observatories, cannot fail to be highly acceptable to the Government which instituted it, and to the public who have paid for these establishments, as it must be most satisfactory to the officers and to their assistants, by whose patient and unremitting labor facts of which the importance is thus recognized have been added to the foundations of meteorological science. The generalization in which M. Dove has applied them is remarkable alike for its extent and its simplicity, and I am glad of the opportunity of enriching this volume with so interesting a document. EDWARD SABINE.

Woolwich, March 17, 1853.

The establishment of meteorological stations in distant parts of the globe had, generally speaking, for its immediate object, so to complete the partial knowledge we already possessed of the phe

* From "Observations made at the Magnetical and Meteorological Observatory at Hobarton, in Van Dieman Island," vol. iii, Introduction.-Phil. Mag., Oct., 1854.

nomena over a considerable portion of its surface, as to enable us to take a general view of their course over the whole globe; the result of those endeavors has even exceeded what was hoped for, as besides the information obtained respecting regions where our knowledge was most defective, fresh light has been thrown on those with which we had supposed ourselves already completely acquainted.

Meteorology commenced with us by the study of European phenomena, and its next principal extension was to phenomena observed in the tropical parts of America. If what is true of Europe were equally true of the temperate and cold zones of the earth in all longitudes, and if tropical America in like manner afforded a perfect example of the tropical zone generally, it would be of little consequence where the science of meteorology had been first cultivated; but this is not the case, and a too hasty generalization has led to the neglect of important problems, while others less important have been regarded as essential and placed in the foremost rank. It was necessary that the science should be freed from these youthful trammels, and this needful enfranchisement has been effected by the Russian and by the English system of observations. Russia has done her part in freeing the meteorology of the temperate and cold zones from impressions derived exclusively from the limited European type; and Eng· land, which by its Indian stations had undertaken for the torrid zone the same task of enlarging and rectifying the views previously entertained, has besides, by its African and Australian stations (Cape of Good Hope and Hobarton), opened to us the southern hemisphere, and first rendered it possible to treat of the atmosphere as a whole. I will now endeavor to show the importance of being enabled to take such general views, selecting as an example the annual variation of the barometer.

The study of the annual barometric variation had long been singularly neglected, while the diurnal barometric variation had had devoted to it an attention quite disproportioned to its subordinate interest in reference to the general movements of the atmosphere. This otherwise incomprehensible mistake is excused by the localities where nature had been first interrogated. As the diurnal variation had manifested itself with great distinctness and regularity in tropical America, it naturally presented itself as an object of interest in Europe also. The annual variation, on the other hand, is inconsiderable, both in Europe and the tropical parts of America; and thus, while atmospheric phenomena were treated simply as facts of which the periodicity alone was to be investigated, without seeking for physical causes, it was natural that a phenomenon, in which opposite effects resulting from two different causes counterbalance each other, should altogether escape notice. It is, perhaps, more remarkable that no surprise

should have been excited when the atmospheric pressure was not found to diminish from winter to summer, with increasing heat.

When, by the labors of Prinsep more particularly, the phenomena of the tropical atmosphere in Hindostan became more known, there was seen to be a great difference between the barometric variation there and in tropical America; inasmuch as the Indian observations showed a decidedly well-marked annual variation. A new error was now fallen into, and it was supposed that the phenomenon did not extend beyond the torrid zone, and that it was an immediate consequence of the periodical change of wind, i. e. of the monsoons. This erroneous view was completely refuted when the barometric relations at the Siberian stations became known; for it was then found, that north of the Himalaya (which in the supposed hypothesis must have formed the limit of the phenomenon), the annual barometric variation was exhibited on a large scale, and over a region so extensive, that the shores of the Icy Sea itself could hardly be assumed as its boundary. A greatly diminished atmospheric pressure taking place in summer over the whole continent of Asia must produce an influx from all surrounding parts; and thus we have west winds in Europe, north winds in the Icy Sea, east winds on the east coasts of Asia, and south winds in India. The monsoon itself becomes, as we see, in this point of view only a secondary or subordinate phenomenon. I have endeavored to establish the reality of the above phenomenon and its climatological bearings in several memoirs; and I must refer for the numerical values to Poggendorff's Annalen, vol. Iviii, p. 177; vol. lxxvii, p. 309; and to the Berichte of the Berlin Academy, 1852, p. 285. I will here embody the results in distinct propositions, in order to show, in connexion therewith, the importance of the bearings of the Hobarton observations.

1. At all stations of observation in the torrid and temperate zones, the elasticity of the aqueous vapor contained in the atmosphere increases with increasing temperature. In the region of the monsoons this increase from the colder to the warmer months is greatest near their northern limit. Hindostan and China present in this respect the most excessive climate. No differences of similar magnitude are found in the southern hemisphere. The form of the curve of elasticity of the aqueous vapor shows, however, a less decidedly convex summit in the region of the monsoons than beyond it, having in that region rather the character of a flattened summit or table-land, the elasticity continuing nearly the same throughout the period of the rainy monsoon. Near the equator the convex curve of the northern hemisphere becomes, first flattened, and then gradually transformed into the concave curve of the southern hemisphere. In the Atlantic this transition takes place in a rather more northerly parallel. In regard to the magnitude of the annual variation, the following rule SECOND SERIES, Vol. XIX, No. 55.-Jan., 1855.

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appears generally applicable in the torrid zone: the annual variation is considerable at all places where equatorial currents prevail when the sun's altitude is greatest, and polar currents when the sun's altitude is least; and inconsiderable wherever the direction. of the wind is either comparatively constant throughout the year, or where it changes in the contrary sense to that above described. At the last-named class of places the rate of decrease in the mean annual tension of the aqueous vapor with increasing distance from the equator is more rapid than in the first class.

2. At all stations in Europe and Asia the pressure of the dry air decreases from the colder to the warmer months, and everywhere in the temperate zone has its minimum in the warmest month.

3. If we compare the annual variation of the pressure of the dry air in northern Asia and Hindostan with the variation in Australia and the Indian Ocean, we shall be satisfied that something more takes place than a simple periodical change of the same mass of air in the direction of the meridian, between the northern and southern hemispheres. From the magnitude of the variation in the northern hemisphere, and the extent of the region over which it prevails, we must infer that at the time of diminished pressure a lateral overflow probably takes place; that it actually does so may be considered as proved for the northern part of the region, by the fact that at Sitka, on the northwest coast of America, the pressure of the dry air increases from winter to summer. It is not probable that the overflow takes place exclusively to the east, it probably occurs also to the west; and on this supposition the small amount of the diminution of the pressure of the dry air from winter to summer in Europe would be caused, not solely by the moderate amount of the difference of temperature in the hotter and colder seasons, but also by the lateral afflux of air in the upper regions of the atmosphere tending to compensate the pressure lost by thermic expansion. As at the northern limit of the monsoon, at Chusan and Pekin, the annual variation of the pressure of the dry air is most considerable, while at the northern limit of the trade wind in the Atlantic Ocean, i. e. at Madeira and the Azores, it is very small, it is probable that there is in the torrid zone also a lateral overflow in the upper strata of the atmosphere from the region of the monsoons to that of the trades.

4. From the combined action of the variations of the aqueous vapor and of the dry air we now derive immediately the periodical variations of the whole atmospheric pressure. As the dry air and the aqueous vapor mixed with it press in common on the barometer, so that the upborne column of mercury consists of two parts, one borne by the dry air, the other by the aqueous vapor, we may well understand that as with increasing tempera

ture, the air expands, and by reason of its augmented volume rises higher and at its upper portion overflows laterally,-while at the same time the increased temperature causes increasing evaporation, and thus augments the quantity of aqueous vapor in the atmosphere, so it naturally follows that the composite result in the periodical variations of the barometric pressure should not everywhere bear a simple and immediately obvious relation to the periodical changes of temperature. It is only when we know the relative proportions of the two variations which take place in opposite directions that we can determine whether their joint effect will be an increase or a decrease with increasing temperature,-whether in part of the period the one variation may preponderate and in other parts the other variation. The following are the results which we are enabled to derive from observation.

5. Throughout Asia, the increase in the elasticity of the aqueous vapor with increasing heat is never sufficient to compensate the diminished pressure of the dry air, and the annual variation of barometric pressure is therefore everywhere represented in accordance with the variation of the pressure of the dry air, by a simple concave curve having its lowest part or minimum in July. The observations in Taimyr Land, at lakonsk, Udskoi and Aiansk, show that this is true up to the Icy Sea on the north, and to the sea of Ochotsk on the east. On the west a tendency towards these conditions begins to be perceived in European Russia in the meridian of St. Petersburg, and becomes more marked as the range of the Ural is approached. On the Caspian and in the Caucasus the phenomenon is already very distinctly marked; its limit runs south from the western shore of the Black Sea, so that Syria, Egypt and Abyssinia fall within the region over which it prevails. Towards the confines of Europe there is almost everywhere a maximum in September or October, the barometric pressure increasing rapidly from July to the autumn. This maximum is followed towards the latter part of the autumn by a slighter inflexion or secondary minimum; it is only beyond the Ural that the curves become uniformly concave, with a single summer minimum and winter maximum, which character they retain throughout the rest of the Asiatic continent, even to its eastern coast. In winter the absolute height of the barometer at the northern limit of the monsoon is very great. The still considerable amount of the annual variations at Nangasaki, and the little difference between the curve of Manilla and that of Madras, show that the region in question extends beyond the eastern coast of Asia into the Pacific Ocean; in higher latitudes, however, its limits appear to be reached in Kamschatka. As the annual variation, which is greater at Madras than at Manilla, is found greater at Aden than at Madras, the western limit of the region would appear to extend far on the African side.