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countries, and the large amount of moisture in the air in winter, and the heavy rainfall, which, by setting free great quantities of latent heat, still further augments and accelerates the outflow.
80. The variations in mean pressure are very slight, and not marked by any very decided regularity in their march through the seasons, at Dublin, Glasgow, London, Paris, and Rome. As compared with Barnaul and Reykjavik their temperature is at no season very different from that of surrounding countries, and the vapour and rainfall are at no time much in excess or defect, but are more equally distributed over the different months of the year.
81. At the Great St Bernard, 8174 feet above the sea, the pressure in summer is 22.364 inches, while in winter it is only 22.044 inches. At Padua, there is scarcely any difference in the pressure between summer and winter. The increase in the summer pressure at the Great St Bernard is no doubt due to the cause already referred to in par. 71 — viz., the expansion of the air upward during the warm summer months, thus raising a larger portion of it above the barometer at the higher station. But at Santa Fé de Bogota, 8727 feet high, near the equator, and where, consequently, the difference between the temperature in July and January is very small, the difference in the pressures of the same months is also very small, being only 0.018 inch. At Guatemala, 4856 feet high, the mean pressure in January is 25.269 inches, and in July 25.247 inches, showing a difference of 0.022 inch.
82. There is a noteworthy peculiarity in the atmospheric pressure at Great Salt Lake City, Utah, United States, which is 4260 feet high. Though the mean July temperature is 790.0 and the January 25°.0, yet the mean atmospheric pressures of these months is 25.645 and 25.880 inches respectively. This winter pressure, when reduced to sea-level, gives the extravagantly high mean of 30.440 inches for Januaryan amount nearly 0.300 inch greater than any other North American station. Those anomalous results are explained by the physical characteristics of the place. Great Salt Lake City
is situated in a vast natural depression in the high table-land of America, measuring 500 miles each way, and from which there is no outlet. The streams which water the country flow into the Salt Lake, and thence pass off by evaporation. Hence, during the winter months, cold air currents flow down into this bowl-shaped region, and there being no escape for them, the cold, dense air they bring with them settles in the lowest levels, thus causing a most unusual winter pressure for places at that height and temperature. The cold air thus accumulated is poured out down the passes which enter the region on all sides, thus causing those terrific blasts of cold, piercing wind which sweep down the gorges, and cut off almost all communication during the cold season.
ATMOSPHERIC PRESSURE, ITS DISTRIBUTION OVER THE GLOBE.
83. The scientific and practical value of a knowledge of the distribution of atmospheric pressure over the globe during the different months of the year is self-evident. Indeed it is impossible to discuss satisfactorily those inquiries which relate to prevailing winds, the varying temperature, and the rainfall over the world, till the isobarometric lines for the months have been laid down. These lines may be justly regarded as furnishing the key to all questions of meteorological inquiry. Some of the more important facts have been long known, in a general way—such as, the belt of low pressure in the equatorial regions, bounded on each side by the high pressures from which the north and south trades flow; the low pressures round the poles; the low pressures in the North Pacific and North Atlantic; and the low summer pressure of Central Asia. But so far as I am aware, no attempt has hitherto been made to state the facts numerically by means of isobarometric lines for the months in the same way as the temperature of the globe has been represented by Humboldt and Dové, by isothermal lines. These charts are offered as the first approximation to the solution of this imimportant physical problem.
84. The enormous labour required to collect monthly barometric observations, and, in not a few cases, to find the means from daily observations—to average these—to correct for daily range-and reduce to sea-level,-is no doubt one of the chief reasons why isobarometric charts have not hitherto been prepared. My leisure hours for a long time have been employed in preparing the materials for the thirteen Isobarometric Charts, for the twelve months and the year. Three of these are given in this Chapter-viz., the Charts for July, January, and the year.
85. The charts are drawn from observations made at 360 places, thus distributed over the globe-167 in Europe, 51 in Asia, 23 in Africa and adjoining islands, 63 in North America, 35 in South America, West India Islands, and Atlantic, and 21 in Australasia and the Antarctic Ocean. Of the European stations, 12 are in Scotland, 14 in England, 27 in Austria, 12 in Italy, 10 in France, 10 in the Netherlands, 9 in Norway, 57 in the Russian Empire, &c. The list might have been largely increased ; thus a larger number might have been given from the Scottish stations, but the 12 given were judged sufficient to represent the mean atmospheric pressure of that country.
86. For the British Islands the means were uniformly taken for the ten years from 1857 to 1866, so that they might be strictly comparable with each other; and the means of several European places are for the same years. In the United States of America the means are uniform for the six years from 1854 to 1859. In selecting the stations, respect was paid to the obtaining of a good mean,—that is, observations for a sufficient number of years, to show as nearly as possible the true mean. At Bombay, for example, from 1847 to 1860, the lowest mean for July was 29.598 inches in 1851, and the highest 29.673 inches in 1853. This regularity in the pressure of the same month in different years is a feature common to all tropical countries, and hence, in such places, a few years were accepted as a good mean. On the other hand, since the mean pressure at Reykjavik, Iceland, during January 1867, was 29.913 inches ; during February, 29.359 inches; and during March, 30.037 inches,—it is evident that a good many years are required to represent the mean of the months. Hence, a very subordinate place, if any at all, has been given to observations from the places at which the observations did not extend over a considerable number of years.*
87. For many means I am indebted to the labours and writings of Buys Ballot, Carl Jelinek, Dové, Quetelet, and Kuppfer, and particularly to Secchi's admirable abstracts which have appeared from time to time in the 'Bulletino Meteorologico.' To the Royal Society of Edinburgh and to Professor C. Piazzi Smyth, Royal Observatory, Edinburgh, I return my most cordial thanks for giving me, at all times, access to their invaluable libraries.
88. In reducing to sea-level, Table IV. was used for all places that did not exceed 800 feet in height. For higher situations the reduction was made by means of Dippe's method, as detailed in Guyot's “Mathematical and Physical Tables,' D, p. 60.
89. In the charts the isobarometric lines are drawn for every tenth of an English inch in the difference of the pressure. The lines representing 30 inches of pressure and upwards are coloured red; and the lines representing pressures lower than 30 inches are coloured blue. Thus the red lines show at a glance those portions of the globe where the pressure is above the average (30 inches). In the centre of Africa, from which there are no observations, and where, consequently, the lines are hypothetical, they are made dotted lines.
90. MEAN ATMOSPHERIC PRESSURE FOR JULY.—It will be seen from Plate I., representing the mean pressure for July,
* The following are a few of the places, with the number of years, for which the means are given :-Sitka, 15; Algiers, 10; Hobart Town, 25 ; St Louis, Mauritius, 13; Bogoslovsk, 26; Nijni-Tagilsk, 21; Barnaul, 19; Nertchinsk, 18; Pekin; 14; Calcutta, 11; Tiflis, 14; Baku, 17; Alagir, 15; Jakobshavn, 10; Reykjavik, 13; Hammerfest, 13; St Petersburg, 19; Archangel, 18; Zlatoust, 28; Lugan, 22; Christiania, Cracow, and Kursk, 27; Brussels, 33; Gand, 26; Geneva, 25; Ahun, 34; Verona, 73; Bologna, 45; Milan, 25; Turin, 74; and most of the Austrian stations from 14 to 18, &c.