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true means deduced from the 24 observations made each day, were 29.906, 29.882, and 29.800 inches. Thus the applied correction was right in one year, 0.003 too great in another, and 0.002 too small in the third year. Still greater error arises in applying the means of one place to the means of single months of another place which is under quite different meteorological conditions. Meteorologists ought only to publish actual observations; or if they correct their observations, they should accompany them with a clear statement of what has been done.
75. Annual Variation.—When it is summer in the one hemisphere, it is winter in the other. In the hemisphere where summer prevails, the whole air, being warmer than in the other hemisphere, expands both vertically and laterally. As a consequence of the lateral expansion there follows a transference of part of the air from the warm to the cold hemisphere along the earth's surface; and, as a consequence of the vertical expansion, an overflow in the upper regions of the atmosphere in the same direction. Hence, in so far as the dry air of the atmosphere is concerned, the atmospheric pressure will be least in the summer and greatest in the winter of each hemisphere. But the production of aqueous vapour by evaporation being most active in summer, the pressure on the barometer will be increased from this cause. As the aqueous vapour is transferred to the colder hemisphere it will be there condensed into rain, and being thereby withdrawn from the atmosphere, the barometer pressure will be diminished; but the dry air which the vapour brought with it from the warm hemisphere will remain, thus tending to increase the pressure. The annual variation is well seen from a comparison of Plates I. and II.
76. In the neighbourhood of the equator, where temperature and moisture differ little in the course of the year, there is little variation in the mean pressure from month to month. Thus, at Cayenne, the pressure in January is 29.903 inches,* and in July 29.957 inches.
* None of the following pressures are reduced to sea-level.
77. At Calcutta, 22° 33' N. lat., the pressure is 29.530 inches in July, and 30.010 inches in January, thus showing a difference of 0.480; and at Rio de Janeiro, 22° 57' S. lat., it is 29.744 inches in January (summer), and 29.978 inches in July (winter), the difference being 0.234 inch. The large annual variation at Calcutta is caused jointly by the great heat in July, and by the heavy rains which accompany the south-west monsoons at this season ; while in January the barometer is high, owing to the northerly monsoon, by which the dry cold dense air of Central Asia is conveyed southward over India.
78. At places where the amount of vapour in the air varies little from month to month, but the variations of temperature are great, the difference between the summer and winter pressures is very striking. Thus, at Barnaul and Irkutsk, both in Siberia, the pressures in July are respectively 29.117 inches and 28.187 inches, and in January 29.807 inches and 28.774 inches, the differences being upwards of six-tenths of an inch. The great heat of Siberia during summer causes the air to expand and flow away in all directions, and the diminished pressure is not compensated for by any material accessions being made to the aqueous vapour of the atmosphere ; and, on the other hand, the great cold and small rainfall of that region during winter cause high pressures to prevail during that season. The same peculiarity is seen, though in a modified degree, at Moscow, St Petersburg, and Vienna.
79. At Reykjavik, in Iceland, the pressure in June is 29.717 inches, and in December 29.273 inches; at Sandwick, Orkney, 29.775 inches, and 29.623 inches; and at Sitka, in what was Russian America, 29.814 inches, and 29.631 inches. In all these places the distribution of the pressure is just the reverse of what obtains in Siberia, being least in winter and greatest in summer. The high summer pressures are due to the cool summer temperature as compared with surrounding countries, thus causing an overflow from these regions in the more elevated parts of the atmosphere. On the other hand, the low winter pressures are due to the comparatively high winter temperatures causing an outflow towards adjoining 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