Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/42

Rh which are more immediately under the sun brings about^a lower pressure, interesting examples of which occur in India, the Malayan Archipelago, and the Mediterranean, Black, and Caspian Seas. In many cases the lines of pressure follow more or less closely the contours of the coasts. Thus the diminution is greater over Italy and Turkey than over the Adriatic and Black Seas. The greatest diminution occurs in Central Asia, where it exceeds 0.200 inch, and the greatest excess round Iceland, where it exceeds 200 inch. It is to the position of Great Britain, with reference to the deficiency of pressure on the one hand and the excess on the other, that the general prevalence of east winds at this season is due. These easterly winds prevail over the whole of Northern Europe, as far south as a line drawn from Madrid and passing in a north-easterly direction through Geneva, Munich, &c. To the south of this line the diminution of pressure is less, and over this region the winds which are in excess are not easterly, but southerly. Crossing the Mediterranean, and advancing on Africa, we approach another region of lower pressure, towards which easterly and north-easterly winds again acquire the ascendency, as at Malta, Algeria, &c. This, in many cases great, variation of the pressure in the different months of the year must be kept carefully in view in deducing heights of places from observations made by travellers of the pressure of atmosphere, by the barometer or the temperature of boiling water. In reducing the observations, it is necessary to assume a sea- level pressure if the place is at a considerable distance from any meteorological observatory. Previous to the publica tion of Buchan s Mean Pressure of the Atmosphere, it appears that a mean sea-level pressure of 29 92 or 30 00 inches was in such cases universally assumed. The mean pressure at Barnaul, Siberia, being 2 9 5 3 6 inches in July, 30-293 inches in January, and 29 95 4 inches for the year, it follows that, by the former method of calculating the heights, observations made in January to ascertain the height of Lake Balkash would make the lake 350 feet too high, and observations made in July would make it 330 feet too low, the difference of the two observations, each set being supposed to be made under the most favourable circumstances, and with the greatest accuracy, being 680 feet. This illustration will serve to account for many of the discrepancies met with in books regarding the heights of mountains and plateaus. Of the periodical variations of atmospheric pressure, the most marked is the daily variation, which in tropical and sub-tropical regions is one of the most regular of recurring phenomena. In higher latitudes the diurnal oscillation is masked by the frequent fluctuations to which the pressure is subjected. If, however, hourly observations be regularly made for some time, the hourly oscillation will become apparent. The results show two maxima occurring from 9 to 11 A.M. and 9 to 11 P.M., and two minima occurring from 3 to 6 A.M. and 3 to 6 P.M. The following are the ex treme variations for January, April, July, and October from the daily mean pressure at Calcutta, deduced from the observations made during six years, viz., 1857-62: A. . P. M. Min. Hour. Max. Hour. Min. Hour. Max. Hour. January April July Inch -023 -020 -019 3 3 3 Inch. + 079 + 070 + 040 10 9 10 Inch. -053 -071 -051 4 4 4 Inch. + 010 + 016 + 029 10 10 10 October ..... -026 3 + 064 9 -047 4 + 018 10 Similarly the maxima and minima at Vienna, with the hour of their occurrence, are as follows : A. fcL I .J J. Min. Hour. Max. ETour. Min. Hour. Max. Hour. January April Inch. -008 -003 6 5 Inch. + 018 + 021 10 10 Inch. -020 -027 3 5 Inch. + 012 + 014 10 11 July + 003 3 + 022 9 -028 5 + 009 11 October -010 6 + 020 10 -015 4 + 008 10 These two illustrations may be regarded as typical, to a large extent, of the diurnal barometric oscillations in. tropical and temperate regions. At Calcutta the amounts are large, and the dates of the occurrence of the maxima and minima very regular from 3 to 4 and 9 to 10 A.M. and P.M. respectively. On the other hand, the oscillations at Vienna are much smaller and more variable in amount, and the dates of occurrence of the critical phases take place through a wider interval, viz., from 3 to 6 and 9 to 11 A.M. and P.M. respectively. Though the diurnal barometric oscillations are among the best-marked of meteorological phenomena, at least in tropical and sub-tropical regions, yet none of these pheno mena, except perhaps the electrical, could be named respecting whose geographical distribution so little is really known, whether as regards the amount of variation, the hour of occurrence of the critical phases, or, particular!} , the physical causes on which the observed differences de pend. This arises chiefly from the want of a sufficient number of ascertained facts ; and to remedy this deficiency, observations have, in the preparation of this present article, been collected and calculated from upwards of 250 places in different parts of the globe, and the data set down on charts. The chief results of this inquiry are the following, attention being entirely confined to the chief oscillation, viz., that occurring from the A.M. maximum to. the P.M. minimum.

The A.M. Maximum. In January this occurs from 9 to 10 in tropical and temperate regions as far as 50 N. lat. ; in higher latitudes the time of occurrence varies from 8 A.M. to noon. In July it occurs from 9 to 10 everywhere only as far as about 40 N. lat; the time at Tiflis (41 42 N. lat.) being between 7 and 8 A.M. In higher latitudes the time varies from 8 to 1 1 A.M., the last hour being general in north-western Europe.

The P.M. Minimum. In January this occurs from 3 to 4 P.M. nearly everywhere over the globe, a few exceptions occurring in north-western Europe, the extremes being 2 P.M. at Utrecht and 6 P.M. at St Petersburg. It is quite different in July, when the time from 3 to 4 P.M. is regu larly kept as far north as about 40 N. lat. In higher latitudes the hour is very generally 5, but at some places it is as early as 4 P.M., and at others as late as 6 P.M. In the northern hemisphere, in summer, the afternoon minimum falls to a greater extent below the mean of the day than the forenoon maximum rises above it, at 82 per csnt. of the stations; but in winter the percentage is only 61. In the southern hemisphere the same relation is observed in the summer and winter months, thus showing that in the summer of both hemispheres the influence of the sun tends to lower the minimum at 3 to 4 P.M. to a greater extent than to raise the 9 to 10 A.M. maximum.

Decrease between Morning Maximum and Afternoon Minimum. Of the four daily oscillations, this is the most important. When the amounts at different places are entered on charts of the globe, it is seen that the amplitude of this fluctuation is, speaking generally, greatest in tho tropics, diminishing as we advance into higher latitudes] greater over the land than over the sea, increasing greatly on proceeding inland ; nearly always greater with a dry than with a moist atmosphere ; and generally, but by no means always, it is greatest in the month of highest 