Pressures of dry AIR, AT DIFFERENT HOURS, AT HALLE AND AT MUNSTER. These quantities differ greatly from those that I deduced from the observations at Apenrade; for in July we find a maximum at Halle a short time after midnight, and the pressure of the dry air diminishes until 8 A.M.; it then again increases, and attains a new maximum at the moment when the sun passes the meridian: the minimum occurs about seven o'clock in the evening; the pressure then increases again. The difference between the extremes does not vary more than 0,9 and 1mm,1; consequently it is one-fourth of that found for Apenrade during the summer. In January there are two maxima, at 10 A.M. and 10 P.M.; and two minima, at 5 A.M. and 2 P.M. The series for Munster seems to indicate analogous laws. It is not probable that, in places where difference of latitudes is so inconsiderable, the phenomenon should present such great differences; for, when we study the barometric oscillations without separately examining the pressure of the air, and that of the vapour of waters, these differences entirely disappear. The anomalies increase when we compare series made at great differences of level. If we would explain the great amplitude of diurnal oscillation in the open sea, by differences of tension, that would suppose a variation in the dew-point of several degrees; now, the observations of voyagers prove that the heat and the tension of the vapour of water vary but little during the day on the surface of the ocean. Admitting the influence of this single cause, we remove other difficulties also. It is probable that the mean pressure of the atmospheric vapour of water, considered in the space of one day or one year, diminishes with the height, according to the laws of the tension of vapours in vacuo. Thus, from an observation made on one point of the vertical, we may deduce approximatively the tension to any height; but it would not necessarily follow that this rule might be applied to every hour of the day taken isolately. As the air opposes the liberation of vapours, it is evident that they are denser in the morning at the surface of the ground than an observation made at a certain height would lead us to suppose. So that, in the plains, the dew-point would be too high, and the pressure of vapour that we might deduce from it will be greater than if it were deduced from measures made at different heights; but, towards mid-day, the ascending current makes the vapours ascend, and the dewpoint then becomes lower than it would have been according to the mean hygrometric state. These facts are a consequence from the observations I made on the Alps. At the sea-shore these laws are modified, for while the ascending current raises the vapours the sea-breeze is continually bringing new. Whatever be the cause of these oscillations, their amplitude can never be so great above as below. Suppose that, in the plains, the barometer has the same height throughout the day, there will be above a variation, the laws of which it would be easy to deduce; for as soon as the presence of the sun above the horizon expands the air the latter ascends, and a part of the atmosphere that was below the elevated station rises above it, whence there arises an increase of pressure until the moment of the greatest heat. From this moment the barometer falls until the moment of the minimum diurnal temperature, which happens next morning. This period combines with that which takes place on the plain, for about mid-day the atmosphere becomes lighter below and above; but this diminution is less sensible above, because of the expansion of the entire atmosphere: the phenomenon, therefore, depends specially on the amplitude of the variation in the plain. The more we ascend the less does the barometer change from twenty-two hours to four hours, and it may happen that the pressure unceasingly increases until the moment of the greatest diurnal heat. In our latitudes this interversion will shew itself at a less height than within the tropics, where the diurnal oscillations are much more powerful. MEAN HEIGHT OF THE BAROMETER. - Like as we have been able to deduce the mean temperature of a place from a few diurnal observations, so also can we conclude the mean height of the barometer for a tolerably long period of time, from a few readings made every day. I have designedly said a tolerably long period, for, if we desire to obtain the mean pressure of the air during the day, the value obtained by means of some observations may differ sensibly from the real mean. The mean barometric height is obtained approximately by taking observations at the morning maximum and the evening minimum. A great many meteorologists only read the instrument at these two moments of the day: I cannot press too strongly upon them to make several other observations in the morning and afternoon; for not only do they obtain the mean pressure more accurately, but their observations may serve for studying the irregular oscillations of the barometer. Further, if in any place an observer is engaged measuring heights by the barometer, two correspondents in the day are not sufficient; and that of mid-day, in particular, always gives differences of level that are too great: but, if observations were made four or five times a-day, we might then calcu late the variation and the mean pressure, and appreciate the extent of irregular variations. The arithmetical mean of three observations made at eighteen hours, two hours, and ten hours, or, again, at nineteen hours, two hours, and nine hours, is sensibly equal to the mean barometric pressure, and the amplitude of the diurnal oscillations may be deduced from it. The barometer attains its mean height about mid-day, generally between mid-day and one o'clock; the moment varies according to the seasons. HEIGHT OF THE BAROMETER ON THE SEACOAST.―This was for a long time thought to be the same in all latitudes. The number of observations not being sufficient to solve the question, we trusted to theoretical considerations; it was said that the conditions of equilibrium of the aërial ocean did not sanction our admitting unequal pressure at different latitudes. It was forgotten that this assumed equilibrium did not exist; for if, in our latitudes, the oscillations due to changes in the weather end by compensating each other, it is not the same within the different zones; the existence even of trade-winds near the equator, and west winds in high latitudes, is a sufficient proof of this. The ascending current that, in the higher regions, is always directed toward the poles, draws along the air of the equator, and the stronger pressure that results from it leads the air of the pole toward the equator, and gives rise to trade-winds. MM. Schouw, Erman, Herschel, Muncke, and Poggendorff, have successively entered upon this subject; and, although the influence of latitude is not as accurately known as might be desired, yet we already possess very satisfactory approximations. The principal results to which we have arrived are the following: 1st. We may admit that, at the sea-coast, the mean atmospheric pressure is 761mm,35. 2d. At the equator, it is not more than 758mm, or a little above. 3d. At the latitude of 10° the pressure increases, and between the 30th and 40th degrees it attains its maximum, for it rises to 762 or 764 4th. Starting from this zone it diminishes, and about the 5th degree of latitude it is no more than 760mm, and in the more northern countries it descends to about 756mm. We here give M. Schouw's table, as it was published by M. Poggendorff in the Comptes rendus de l'Académie des Sciences, t. ii. p. 573. 1836. |