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of higher latitudes the air is more condensed in the lower 

 strata, t'nis leaving a less pressure of air at great heights. 

 It fjllows that the steepest barometric gradients for the 

 upper currents of the atmosphere will be formed during 

 the coldest months of the year. At Bogota, 8727 feet in 

 height, where the temperature is nearly uniform through- 

 out the year, the mean pressure for January and July are 

 22'048 and 22058 inches. On the other hand, at Mount 

 Washington, 6285 feet high, where the January and July 

 mean temperatures are (y'\ and 48^2, the mean pressures 

 for the same months are 23'3g2 and 23-87; inches. Simi- 

 larly at Pike's Peak, 14,151 feet high, the mean tempera- 

 tures are 3^'i, and 39'7, and the mean pressures I7'493 and 

 i8o6g inches; and since the sea-level pressures in the 

 region of Pike's Peak arc nearly o'5oo inch higher in Janu- 

 ary than in July, it follows that the lowering of the pres- 

 sure on the top of Pike's Peak due to the lower tempera- 

 ture of January is upwards of I'ooo inch. From the 

 greatly steeper barometric gradients thus formed for 

 upper currents during the cold months of the year from 

 equatorial to polar regions, these currents attain their 

 maximum strength in winter and converge upon those re- 

 gions of the earth where the mean temperature is lowest. 



As is now well known, atmospheric pressure in summer is 

 lowest in the central regions of the continents of Asia, 

 Africa, and America ; and highest in the Atlantic between 

 Africa and the United States, and in the Pacific between 

 the United States and Japan, the absolutely lowest being 

 in Asia, where temperature is relatively highest with re- 

 spect to the regions immediatel)' surrounding, and ab- 

 solutely lowest in the Atlantic, which is most completely 

 surrounded with highh'-heated continental lands. Again, in 

 winter the lowest atmospheric pressures are found in the 

 north of the Atlantic and Pacific Oceans, where temperature 

 is relatively highest, latitude for latitude ; and the highest 

 pressures towards the centres of the continents, some 

 distance to southward of the regions where at this season 

 abnormally low temperatures are lowest. 



The causes which bring about an unequal distribution 

 of the ntass of the atmosphere are the temperature and the 

 moisture considered with respect to the geographical dis- 

 tribution of land and water. Owing to the different 

 relations of land and water to temperature, the summer 

 temperature of continents mtich exceeds that of the ocean 

 in the same latitudes ; and hence results the abnormally 

 high temperature of the interior of Asia, Africa, America, 

 and Australia during their respective summers, in conse- 

 quence of which the air becoming specifically lighter as- 

 cends in enormous columns thousands of miles in diameter. 

 Winds from the ocean set in all round to take the place 

 of the air thus removed, raising the rainfall to the annual 

 maximum, and still further diminishing the atmospheric 

 pressure. On the other hand, since in winter the 

 temperature of the continents and their atmosphere falls 

 abnormally low, the air becomes more condensed in the 

 lower strata, and pressure is thereby diminished in the 

 upper regions over the continents. Upper currents set 

 in all round upon the continents, and thus the sea-level 

 pressures become still further increased. Hence the ab- 

 solutely highest mean pressure occurring anj'w-here on the 

 globe at any season, about 30'5oo inches, occurs in Africa in 

 the depth of winter. 



Now observation conclusively proves that from the 

 region of high pressure in the interior of Asia in winter, 

 from the region of high pressure in the Atlantic in summer, 

 and from all other regions cf high pressure, the winds blow 

 outwards in all directions ; and that towards the region of 

 low pressure m Asia in summer, towards the region of low 

 pressure in the north of the Atlantic in winter, and to- 

 wards all other regions of low pressure, whenever and 

 wherever they occur, the winds blow in an in-moving 

 spiral course. 



-Since enormous misses of air are in this way poured 

 into the region where pressure is low without increasing 



that pressure, and enormous masses of air flow out of the 

 region where pressure is high without diminishing that 

 pressure, it is simply a necessary inference to conclude 

 that the masses of air poured all round into the region of 

 low normal pressure do not accumslatc over that region, 

 but must somehow escape away into other regions ; and 

 that the masses of air which flow outwards on all sides 

 from the region of high normal pressure must have their 

 place taken by fresh accessions of air poured in from 

 above. Keeping in view the law of the barometric gradient 

 as applicable to all heights of the atmosphere, it is evident 

 that the ascending current from a low-pressure area, the 

 air composing which is relatively warm and moist, will 

 continue its ascent till a height is reached at which the 

 pressure of the air of the current equals or Just falls short 

 of the pressure over the surrounding regions at that high 

 level. On reaching this height, the air, being no longer 

 buoyed up by a greater specific levity than that of the sur- 

 rounding a'r, ceases to ascend, and thereafter spreads itself 

 horizontally as upper currents towards those regions which 

 offer the least resistance to it. The overflow of the upper 

 currents is thus in the direction of those regions where 

 pressure at the time is least, and this again we have seen to 

 be towards and over that region or those regions the air of 

 which in the lower strata of the atmosphere is colder 

 and drier than that of surrounding regions. 



The broad conclusion is this : the winds on the surface 

 of the globe are indicated by the isobaric lines showing 

 the distribution of the mass of the earth's atmosphere 

 near the surface, the direction of the wind being from 

 regions where pressure is high towards regions where 

 pressure is low, in accordance with Buys Ballot's law. 

 On the other hand, the low-pressure regions, such as 

 the belt of calms in equatorial regions, the interior 

 of Asia in summer, and the north of the Atlantic and 

 Pacific in winter, with their ascending currents, and rela- 

 tively higher pressure at great heights as coinpared with 

 surroundmg r^egions, point out the sources or fountains 

 whence the upper currents flow. From these sources the 

 upper currents spread themselves and flow towards and 

 over those parts of the earth where pressure is relatively 

 low. These directions are, speaking generally, from equa- 

 torial to polar regions ; but more particularly towards and 

 over those more restricted regions where in the lower 

 strata of the atmosphere the air is colder and drier than 

 in neighbouring regions, such as the Atlantic between the 

 United States and Africa in summer, and Central Asia in 

 winter. 



This view of the general movements of the upper 

 currents of the atmos^ihere is in accordance with the 

 observations which have been made in dift'erent parts of the 

 globe on the motions of the cirrus cloud, and with obser- 

 vations of the directions in which ashes from volcanoes 

 have been carried by these upper currents. In further 

 corroboration of the same views, reference may be made 

 to the researches made in recent years, particularly by 

 Prof. Hildebrandsson and Clement Ley, into the upper 

 currents of the atmosphere, based on observations of the 

 movements of the cirrus cloud in their relation to the 

 cyclones and anticyclones of north-western Europe. 



An important bearing of cyclonic and anticyclonic 

 areas on the distribution of temperature may be here 

 referred to. The temperature is abnormally raised on the 

 east side of cyclonic areas and abnormally depressed on 

 their west sides; but, on the other hand, temperature is 

 abnormally raised on the west sides of anticyclonic areas, 

 and depressed on their east sides — the directions being 

 reversed in the southern hemisphere.' Since the tempera- 

 ture is lower in the rear than in the front of a cyclone, it 

 follows that, relatively to the sea-level pressures, pressure 

 will be lower in the upper regions in the rear of a cyclone 

 than in front of it, a result which the Ben Nevis observa- 



