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appreciable movement could be observed. All this lessens the value of the observations on motion of clouds.

Generally it is seen, that the clouds move from the same direction as the air near the surface of the earth, which would lead to the conclusion that the lower strata of clouds were those observed.

As to the higher clouds, the cirri, as far as known, they move generally from the west, except in the polar regions.

Considering all this, as well as the fact that the motion of clouds is recorded in this work for very few places outside of the United States, I shall not consider the subject in the further deductions, leaving to every one interested to draw his own conclusions from the tables and the map, Plate 1.

The most important works in meteorological science in the last twenty-two years are devoted to the proof of the mutual dependence of atmospheric pressure and winds.

It has for a long time been admitted that in the belts of the trade-winds the air moves from the regions where pressure is high (the polar limits of the trades) towards the low pressure of the equatorial regions. The phenomena here were so simple and regular that the explanation was very easy. In the case of the tropical hurricanes it was also generally admitted that the wind blew towards the low pressure in the centre of the storms. The meteorological phenomena of the temperate and polar regions are much more complicated, and the causes of them less easily detected.

It was Prof. Buys-Ballot who proved the general dependence of the winds on the

pressure of the air. In its original enunciation, his celebrated law of the winds declares that the winds will blow from the region where the barometer is above the mean towards that where it is below, and will be deflected 60° to 80° towards the right, owing to the rotation of the earth. He subjected this law to a severe practical test in using it in the system for prediction of storms which had been established at that time in the Netherlands. Buys-Ballot's law of the winds is now very generally accepted, though in a somewhat modified form, viz. : the wind blows from a region of high pressure towards one of low pressure, anl is deflected to the right owing to the rotation of the earth. In 1853, Prof. Coffin arrived at a very similar conclusion, saying, “that in the northern hemisphere a wind arriving from its mean direction always finds the point of maximum pressure on its left, and the minimum to its right; while the reverse is true in the southern hemisphere. There seem to be no exceptions to this law.” He further states (Proceedings of American Association, 1853, p. 88) that the deflection in this case is 65°; that is, very near to that found by Dr. Buys-Ballot. Even before Professor Coffin, Espy expressed similar views, as seen in his “Philosophy of Storms” and “Meteorological Reports.” Very likely the views of the American meteorologists were too much in advance of their time to be generally accepted. When Dr. Buys-Ballot published his law of the winds, meteorology had made much more progress, so as to render such views more easy of acceptance.

This law applies to storms as well as gentle winds, to single hours of observations as well as to monthly and yearly means.

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Buchan has rendered a great service to meteorology by extending Buys-Ballot's law to the general phenomena of the winds of the globe. He collected a great deal of information as to the mean pressure of the air, and drew isobaric lines, i. e., lines of equal pressure of the air reduced to sea-level, and by considering the prevailing winds he proved that they generally followed Buys-Ballot's law. As this work, “Mean Pressure and Prevailing Winds of the Globe,” is very important in the discussion of the winds, I make the following extracts from it:

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Distribution of Atmospheric Pressure in December, January, and February.

“In these months the highest pressures are grouped over the land of the Northern Hemisphere, and the larger the extent of land, the

pressure. The area of high barometer (thirty inches and upwards) embraces nearly all of Asia, all Europe south of the North and Baltic Seas, the North Atlantic between 15°-45° N., the West Indies, North America except the North and Northwest, and the Northern Pacific between 8o and 21° N. There are also two regions of high pressure of comparatively small extent, the one in the South Atlantic, the other in the South Pacific.

“The regions of low pressure are: the northern part of the North Atlantic and North Pacific, including portions of the continent adjoining; the belt of low pressure in the equatorial region, towards which the trade-winds blow, and the remarkable depression in the Antarctic region which is probably subject to little change throughout the year.

" In March the pressure diminishes over Asia, the middle and south of Europe and the United States. Everywhere else except in the tropics it is rising. This rise of pressure is most apparent in the temperate regions of the southern hemisphere. In the north of the Atlantic it is rapidly rising, the average pressure in Iceland now being 29.609 inches, thus showing an increase of 0.34 inch in comparison with January.

“In April, the heavy lines indicating a pressure above the average have all but left Asia, Europe, and the United States, and the isobars of 30 inches bound a belt of high pressure, which completely encircles the globe in the south temperate

Pressure continues to rise in the north of the Atlantic, and to the north of North America. And it is probable that a space of high pressure (at least 30 inches) completely encircles the north pole. In this month pressure is more equally distributed than in any other month; for, except the Antarctic Ocean, it scarcely rises anywhere above 30.1 inches nor falls below 29.8. In May, in North Europe, in Greenland, and in the north of North America, pressure attains the maximum of the year. Pressure continues to increase in the south temperate zone, and the isobar of 30.1 now nearly encircles the globe. At this time the highest pressure in the southern hemisphere occurs in the S. E. of Australia, where, at Deniliquin, it is 30.185 inches. Pressure is rapidly falling over Asia and the United States.

“In June, July and August, pressure falls in the central regions of Asia to about 29.5. In this season this diminution of pressure, which may be regarded as entirely

zone.

storms,

determining the summer climate of Asia, reaches its lowest point. Pressure falls also in the interior of North America, where, at Salt Lake City, it is only 29.7 inches. The annual maximum of the south temperate zone is attained in these months. The isobar of 30.1 goes entirely round the globe, and a still higher pressure prevails over South Africa, and the portions of the ocean immediately to the west and east of it. In these months the arrangement of the isobars may be regarded as being, generally speaking, reversed from that of December, January and February, and in this respect a comparison of these two groups of months is very instructive.

“From this period, pressures increase over the continents of the northern hemisphere, and diminish over the south temperate zone, till the distribution of pressure is regained which has been shown to prevail during the winter months.

“In September and October an interesting feature of these lines is a very rapid diminution of pressure, indicated as taking place in the north of the Atlantic and surrounding regions. This is the season of the year when the first great decrease of temperature takes place, which is accompanied by heavy rains and furious

The increase of pressure in Sweden in October, taken in connection with the simultaneous decrease in Greenland, Iceland, the north of Norway, and the British Islands, is interesting as bearing on the transport of masses of the atmosphere from one region into another.

“ In November, pressure rises considerably over the continents of the northern hemisphere, and falls in the south temperate zone.

And the belt of low pressure in the equatorial regions may be regarded as passing completely around the globe. This belt, towards which the trades on each side of the equator blow, does not occur in the summer months in the Indian Ocean; but, on the contrary, there is a continuous diminution of pressure northward, from Australia and Mauritius to the interior of Asia. It will be seen that in November, as compared with October, the isobars have advanced a little northward from the British Islands to Iceland, and eastward from Baffin's Bay to Iceland, thus indicating a general increase of pressure over the north of the Atlantic and regions adjoining. Coincident with this increase of pressure, there occurs a diminution of pressure to the southeast of it, including Austria, Italy, and countries adjoining the Mediterranean; and in the Atlantic to the south of it, from about latitude 15°-45° N. Probably these extensive oscillations of pressure are part of a general movement of the atmosphere, which, in one of its manifestations, has been generally known to meteorologists as the great November wave, but of which no very satisfactory account has yet been given.” (Buchan, p. 577-579.)

WINDS within, or near, a space of Low Pressure.—“Of this class, the best example is the low pressure which prevails in the north of the Atlantic and adjoining regions in the winter months. This region of low pressure is bounded to the S. W. by the high pressure of North America, to the S. by the high pressure in the Atlantic, about 30° lat. N., to the S. E. by the high pressure in the interior of Asia. In January, the difference between the average pressure of Iceland and the interior of Asia is fully an inch.”

“It is seen from the charts that in Baffin's Bay and east of the Rocky Mountains,

1

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 pres

The low pressure in the equatorial regions, towards which the trades blow, is an illustration of the same principle.

“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

sure.

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.

“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

of low pressure. “ 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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July, 1875.

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