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as far south as 40° lat., the winds are N. N. W., N. W., and W. N. W. Crossing the Atlantic, winds in the British Islands, in France, and the north of Germany, from the W. S. W. to S. W.; in Denmark, S. S. W.; near Bergen, in Norway, S.; and at Christiansund and Hammerfest, S. S. E. The relation of these winds to the isobaric lines is the same as that which is illustrated by the winds in storms, in their relation to the isobaric lines of these storms. This has been already stated in a paper by the author, published in the Transactions of the Royal Society of Edinburgh, Vol. XXIV. Part I. p. 201, in the following words: The wind in storms neither blows round the centre of least pressure in circles, or as tangents to the concentric isobaric curves, nor does it blow directly towards that centre. It takes a direction intermediate, approaching, however, more nearly to the direction and course of the circular curves than of the radii to the centre,' Or, according to Dr. BUYS-BALLOT, the angle is not a right angle, but from about 60° 80°. This relation is usually called 'BUYS-BALLOT'S LAW OF THE WINDS." "Another well-marked depression is the low summer pressure in the interior of Asia; with reference to which it is seen from the charts that the winds of Eastern Europe and Western Asia are from N. W. to W. N. W. and W.; at Ceylon, S. W.; at Shanghai, S. E.; and on the Sea of Okhotsk, N. E.; whilst in the interior, calms generally prevail."

“The behavior of the winds, as regards the low pressure of North America, is exactly similar to that of the winds in Asia at this season. In all these cases the wind appears to flow round and in upon the space where pressures are low. Even in those instances where the depression over a limited space is comparatively small, such as in Australia during the summer months, the winds observe the same course with respect to it."

"A well-known and remarkable diminution of pressure is that of the Antarctic regions; and though, except in Tasmania and the south of New Zealand, observations are wanting at particular points for a sufficiently long time to give good averages, yet the concurrent testimony of sailors and the inhabitants of these regions all goes to show that, at least on the outskirts of the region, winds are chiefly N. W. or W. N. W.-that is, they appear to flow in upon the space of low presThe low pressure in the equatorial regions, towards which the trades blow, is an illustration of the same principle."

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“WINDS within, or near, a space of High Pressure.-The most prominent illustration of this is the high pressure in the interior of Asia in winter. It is seen from a single glance at the charts that the winds flow out of this space in every direction. The same outflow is seen with respect to the less strongly marked, but still very distinct space of high pressure in North America; owing to the large number of stations available here, this principle is amply illustrated.

"The next most noteworthy area of high pressure occurs in summer between Africa and North America, out of which also the charts show the winds blowing in all directions towards and round upon the surrounding low pressures.'

"The following mean pressures, in inches, at 32° and sea-level, occur in Australia in June: At Brisbane, Queensland, 30.062; Sydney, 30.116; Melbourne, 30.178; Adelaide, 30.132; Freemantle, 30.121; and at Deniliquin, in the interior, on a

1 For Prof. Coffin's determination of this angle, as 65°, see page xxv.

branch of the Murray River, 30.217. Hence a higher pressure occurs at this season (winter) in the interior, and it may be inferred that it is greatest in the southern portion of the interior. The prevailing winds are these: At Brisbane, S. S. W.; Sydney, W. by N. W.; Melbourne, N.; Adelaide, N. E. by N.; Freemantle, N. E. by E.; in other words, the winds blow out from this space of high pressure.

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"This behavior of the winds with respect to spaces of high pressure differs in no respect from what occurs on particular days on which the isobaric lines present the same conditions of pressure. Mr. FRANCIS GALTON first drew attention to this peculiarity, under the name of Anticyclones, by which name he intended to convey the idea that in cases of high pressure occurring over a limited area, the course of the winds is exactly the reverse of what is seen to prevail in cyclones in which the winds blow round and in upon a space of low pressure.'

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"The outflow of the air from a region of high pressure, and the inflow upon a region of low pressure, appear to be reducible to a single principle, viz., the principle of gravitation. Given as observed facts the differences of pressure, it might almost be predicted, before calculating the averages, what the prevailing winds are. Indeed, so predominating is the influence of gravitation, that it may be regarded as the sole force immediately concerned in determining the movements of the atmosphere. If there be any other force or forces which set the winds in motion, their influence must be altogether insignificant as compared with gravitation.” (Buchan, p. 581 to 583.)

This last passage of Buchan may be more distinctly expressed: in the action of gravity in restoring the equilibrium disturbed by unequal temperature. With a uniform temperature over the whole earth, there would be no wind. In illustration of the dependence of the wind on the difference of pressure, the map of isobars, Plate 14, as well as Plates 2, 4, 5, 6, and others, should be consulted.

Having given the above examples of the manner in which the winds are affected by atmospheric pressure, it is necessary to account for the origin of areas of high pressure, out of which, it is seen, the winds flow.

It must be said that this question is one of the most difficult in meteorology, and far from having received an entire solution.

As the tropical regions present the meteorological phenomena in the simplest form, it is best to begin with them. It has been known for a long time, that above the lower current of the air of the trade winds, flowing in the lower latitudes of the northern hemisphere from N. E. or E. N. E., there exists an upper one from about W. S. W. The existence of this current was proved by the movement of the highest (cirri) clouds always from some westward point, from the strong westerly winds on high mountains in the trade-wind region (the Chimborazo and others in equatorial South America, the peak of Teneriffe, etc.), from the transport eastward of ashes from the eruption of the volcanoes on the island of St. Vincent, (West Indies), and Cosiguina (Central America), and also from the direction of the smoke of very high volcanoes of the tropics. The supposition was then made, that there was a powerful ascending current over the belt of calms and rains near the equator, and that the air thus ascended flowed in the upper regions of the

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atmosphere, in a direction contrary to the trade-winds, towards the polar limits of the latter, or to about 30° N. lat. and descended there.

Then the same principle was extended to dry, hot continental areas, where a powerful ascending current must exist on account of the heating by the sun, and this was proved by the great decrease of pressure in summer time.

Buchan extended the idea af ascending and descending currents further, supposing there was an ascending current over every area of low barometer, not only near the equator and on warm continental areas, but also in high latitudes, as on the North Atlantic, the North Pacific, etc. This air, he supposed, descended over areas of high pressure, as for example those existing in winter in Northern Asia and North America. Thus the supposition is, that the air flowing out of areas of high barometers, to a certain extent, comes from above, and again where the barometer is low, air ascends and flows in the higher strata, towards areas of high

pressure.

I must repeat here, that this is a supposition, though a very plausible one, and that the actual facts which would prove the existence of such upper currents, with the exception of the so-called counter-trades in the region of the trade-winds, are very scarce. To these principles I would refer the direction of the wind at Dodabetta in the Neilghiris, in Southern India (above 8000 feet), which is nearly opposite to that observed in the lower strata in Central India, being from the N. W. in summer, that is, from the heated regions of the Punjab, where pressure is very low. In the lower regions, the winds on the contrary are S. E. and S., that is, the air is flowing towards Punjab. Another remarkable fact is the strong, constant, and warm W. wind observed in winter on some mountains near Lake Baikal. At that time of the year, the air is generally calm in lower regions, the cold intense, and pressure high. This west wind of the higher regions would thus seem to be a compensating current, flowing perhaps from Iceland towards the region of highest pressure of Eastern Siberia,

The observations on two of the highest peaks of the Rocky Mountains, above 14,000 feet, have failed to show an upper current of air blowing in a direction different from the lower one. As we have said before, our information as to upper currents is very scanty, and thus great caution seems yet necessary in drawing conclusions.

On the other hand, the influence of pressure on the winds near the surface of the earth is so well authenticated and reliable that we need not hesitate to base further conclusions on it.

The greater part of the earth being covered with water, we can first consider what would be the case if there were no intervening continents. What in this case would be the normal arrangement of pressure on the oceans? A belt of low pressure near the equator, a belt of high pressure at about 30° north and south, and a belt of low pressure about from 60° to 65°, after which the pressure would rise again towards the pole. This gives us three systems of winds at the surface of the earth, easterly (polar) in the lowest latitudes, westerly (equatorial) in the middle latitudes, and again polar in the highest latitudes, in each hemisphere. A

reference to the maps shows that, in the main, such is the actual arrangement of pressures on the oceans and on parts of the continents.

It is easy to see that this is the general conclusion arrived at by Prof. Coffin in his "Winds of the Northern Hemisphere." The main result is thus the same, the study of the winds, alone having shown that this is the case in a great part of the globe, while what we have said as to the pressure of the air shows at least the proximate cause of the prevailing winds. In how far this normal arrangement of winds is disturbed by geographical features, especially by the influence of the continents, will be shown later.

A further condition is the yearly movements of the belts of high and low pressure with the change of seasons. When the sun is in the zenith over the northern hemisphere, the seas under it will be more heated than the southern seas, and the equatorial belt of low pressure, which is also on the seas, the belt of highest temperature, will move northward. Owing to the great specific heat of the water, and consequently to the longer time it takes to cool, this northward movement will continue nearly to the end of the summer. On the other hand, the belt of low pressure in the higher latitudes will also move northward as the temperature rises near the poles, and the storm-tracks can take a more northerly course. The belt of highest pressure between the two of lowest must also take a more northerly position, as the air flows both north and south out from it. There can be no doubt that it holds an intermediate position between the two.

When the sun is in the zenith over the southern hemisphere, the reverse takes place: the equatorial belt of lowest pressure recedes southward, and also that in higher latitudes of the northern hemisphere, as the polar regions are so much cooled that the condensation of vapor there cannot sustain great barometric depressions. These normal or ideal conditions are realized to some extent on the surface of the present oceans, and are the more striking, the larger the bodies of water arc. Generally the southern hemisphere has meteorological conditions which approach more nearly to the normal conditions than the northern. Thus, it will be seen by reference to the map of the isobars that the high pressure in about 30° really encircles the globe in the southern hemisphere, while in the northern, the pressure is highest in January at about latitude from 50° to 53° N. in Asia, and in July the pressure is very low, about 30° L. N. on the same continent. Again the low pressure about from 60° to 65° encircles the globe in the southern hemisphere, the difference of pressure under the different meridians not being great, and further south (especially from 70° to 78°) somewhat higher pressure and easterly winds are found. In the northern hemisphere, on the contrary, the lowest pressure is found on two elliptical spaces, in the Northern Atlantic, about Iceland, and in the Northern Pacific, about the Aleutian Islands, that is, where a great extent of water prevails at about 60°, and the ocean is abnormally heated by currents of warm water.

We thus see that at a distance from the influence of water, the above-stated normal conditions are very much interfered with.

If the earth consisted mainly of continents without intervening oceans, very different conditions would prevail. As continents are more rapidly heated than oceans, temperature would be highest very soon after the passage of the sun

through the zenith of a parallel. The greatest heat in our summer would be about the Tropic of Cancer, in our winter about the Tropic of Capricorn, and this would also be the belt of low pressure at that time. The S. E. trade would cross the equator into the northern hemisphere when the sun has a northern declination, and the N. E. trade follow into the southern hemisphere during the other half of the year, giving a variation of the inner limits of the trades of perhaps 40°, instead of the 10° or 12° which are now observed. Further, as dry continents cool also more rapidly, the cold in the polar region of each hemisphere during the winter would be more intense than now, extending to the whole polar region, and coinciding with a very high pressure.

These hypothetical conditions are much more imperfectly realized than those I have sketched before, as the extent of continents is much less than that of oceans. The nearest approach to realization is on the greatest continent, that of Asia, where the highest pressure of winter is a little north of 50° N. If it is not found further north, it is because the continent does not extend much beyond 73° N. In summer, on the contrary, we find the highest temperature in N. W. India between 30° and 35° N., and also the lowest pressure there and in N. China.

The larger the continent the more it approaches to the ideal conditions I have supposed. In Africa, for example, there is a belt of lowest pressure in summer at about 17° N., and the highest temperature is probably still more to the north.

The narrower continents of North and South America are more under the influence of oceans than Africa,

As already seen, the highest mean pressure on the surface of the globe is found. in winter on the Asiatic continent. It is necessary to mention here a feature of the climate of this continent, explained by geographical conditions, which has a great influence on the winds, namely, the steadiness of pressure in winter. Pressure is so constant here that, though the barometrical range generally increases with latitude, it is not greater at Jakutsk in N. E. Siberia, under 62° N. L., than in Vienna in Central Europe, Lat. N. 48°, or even in St. Louis in North America, under 39° N. L. The coldest and heaviest stratum of air over Eastern Siberia is prevented from flowing towards the south and east, where pressure is low, by the intervening mountains and plateaus, from 3000 ft. to 5000 ft. high. So long as the cold of winter continues, pressure must, therefore, be high over the cold region of Northern Asia. As it is low in the Pacific Ocean and the equatorial regions, air will flow there from the region of high pressure above the mountains and plateaus. But, as above said, the coldest and heaviest lowest stratum cannot flow towards the Pacific on account of the intervening heights; the quantity of air moving in this direction will not be great enough to supply the deficiency. Thus pressure being lower the whole winter in the S. and E., the winds should be regular from the N. and W., and this is really the case.

On the whole southern and eastern slope of Asia we see a mutual reaction of continental and oceanic influences-the great monsoons. The Europeans were first made acquainted with the regular change of wind and weather in India through the campaigns of Alexander the Great. Not only did the Greeks see this change themselves, but they also learned from the natives with how great a regularity.this

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