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epochs of the maxima and minima, and in the shapes of the curves. Generally speaking, for all places situated on open continental plains, the night variation (i.e., from M2 to m1) is small, as at
Kimberley; and much greater for maritime situations, such as Greenwich and Daressalam. Also on, or near, mountain summits such as Agustia (6,200 feet), and Great St. Bernard (8, 100 feet), m.
is as deep as or deeper than m2, while in deep valleys m, may be of no great depth, as at Córdoba, or abortive, as at Klagenfurt in the
Alps, and m2 very pronounced, as compared with stations on the level in the same latitude. Thus, for example, from M1 to m2 at
Greenwich is 024 inch, whereas the corresponding range at Klagenfurt is 054 inch.
The actual epochs of the maxima and minima vary between moderate limits, and so also do the gradients. The second maximum
is not, anywhere, far from 10 or 11 o'clock in the evening, and varies to no great extent throughout the year. In the summer, however, in the case of deep valleys, the early morning minimum tends to vanish (Fig. 3), and hence the second maximum tends to extend itself
into the first, which is, on the whole, over the earth's surface generally, nearly half a day later. This fact is strikingly shewn throughout the summer at Klagenfurt, and is also plain enough at Córdoba. The first maximum, on the other hand, varies largely, averaging 8 a.m. at Klagenfurt, 9 a.m. at Córdoba and Kimberley, 10 a.m. at Greenwich and Agustia, II a.m. at some places in the west of Europe, and later still on some mountain summits in the north temperate zone. Also as the seasons vary, the time of the morning maximum at any one station will vary. Thus at Kimberley, which is fairly representative, the time of M1 varies from about 8 a.m. at midsummer to nearly 10 a.m. at midwinter, consequently following the sunrise. Ón considerable mountains, however, the circumstances are exactly opposite: M, coming later as the sun gets nearer the zenith, so that in some cases M1 almost or quite merges into M2, as, for example, at Great St. Bernard, where the summer oscillation of the barometer resolves itself into a single wave, with only one maximum and one minimum in the day. Buchan has drawn a very instructive set of curves for Mount Washington, shewing the average diurnal oscillations at four different altitudes during one June. It appears from these that the time of M1 falls about 8 a.m., at 2,900 feet, about 10 a.m. at 4,060 feet, about 11 a.m. at 5,500 feet, and about noon at 6,300 feet, whereas there is very little difference in the respective times of the other phases. Incidentally also his curves shew that m1 gets deeper and m, shallower with increasing altitude. It is curious that at some places close to sea level in N.W. Europe the diurnal oscillation in summer is almost exactly like that on high peaks, namely, M1 is retarded until it almost, or quite, merges into M2. In the winter, however, the mountain and valley curves are much more nearly like those of the plains in the same latitude. (Fig. 4.)
According to Sir John Eliot, the amplitude of the night oscillation in India generally is least from April to July, when days are longest, the temperature and its diurnal range excessive, and the air very dry. He gives also the average times of the different phases as follows:
I give in a Table the average monthly ranges of the barometer from one phase to the next at Kimberley, and also the difference between the mean heights of the maxima and minima, together with the amount by which m1 falls below the mean (Table 2).
One very remarkable feature in this Table is that the difference between the mean monthly heights of M1 and M2 is practically constant throughout the year. The differences between the mean
monthly values of m, and m2, on the other hand, are twice as great in the summer as they are in the winter. Also the mean monthly differences between any two other given phases vary considerably month by month.
The fact that there is some sort of diurnal oscillation of the barometer has probably been known for more than two centuries; but it is only within the last century that any really scientific effort has been made to understand it. In 1666 Dr. Beale, an old Cambridge man, observed that generally, in settled and fair weather, the mercury stands higher than it does during rain or storm; and often, both in winter and summer, is higher in the colder mornings and evenings than in the warmer mid-day. No great advance upon this seems to have been made for many years (excepting that the fact of the diurnal oscillation impressed itself gradually upon observers), chiefly, perhaps, because of the imperfections of barometers in those days. For it was not until 1738 that Orme invented his method of thoroughly boiling the mercury in the barometer tubes. Indeed, so little attention was paid to the phenomenon that there is not a single paper devoted to it in the Philosophical Transactions, at any rate, down to the beginning of the 19th century. Dalton, in the first edition of his Meteorological Observations and Essays (1793), does not so much as allude to it, though he deals at some length with barometric variations from day to day. In his second edition (1834) he mentions it, without apparently suspecting that the oscillation is semi-diurnal, and ventures upon an explanation of the cause of it. Even Harvey, in his otherwise excellent Meteorology (1845), wastes no words over it at all. Meanwhile Hudson (1832) had made a series of hourly observations at the rooms of the Royal Society, and determined the mean diurnal curve with a considerable degree of exactitude. It was, however, only within the last 50 years that the matter was seriously taken up, during which time a number of theories of undoubted merit have been propounded. One or two of these I propose to mention as illustrating the progress of meteorological methods.
Here is Sir John Herschel's account of the barometric oscillation as it was regarded in his day :
“(165.) The periodic fluctuations of the barometer are annual and diurnal. The consideration of the former will enable us to form a neater conception of the mode in which the latter arise. When it is summer in one hemisphere it is winter in the other. Hence the air generally incumbent on the heated hemisphere is dilated, and expands both upwards and laterally not only by its own increased elasticity, but also by the increased production of vapour. It therefore not only encroaches on the other hemisphere by lateral extension, but, what is far more influential, flows over upon it. In order to perceive clearly the nature of the process, we must separate in idea the aqueous and aerial constituents of the portion of the atmosphere so transferred. The generation of the former goes on in the heated hemisphere, and replaces, in part at least, the loss of pressure arising from the transfer of air, while in the other the excess