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change took place; how in all this region the winter was the dry, clear time of the year, and summer the rainy season. The navigators of the sixteenth and seventeenth centuries knew that the monsoons extended much further east than India-to the Indo-Chinese Peninsula, the Sunda Islands, and Southern China.

The cause of the monsoons is this: in our winter the continental regions of Asia are cooler than the surrounding seas, and pressure is higher. The air flows from these towards the equatorial calm-belt in the Indian Ocean, and towards the region of low pressure in the Northern Pacific, as a N. E., N., N. W. or W. wind. As the pressure is continually lower on the seas than on land at this season, this flow of air is very constant. As the air comes from the interior of the continent, and generally also from higher latitudes, i. e., from colder regions, the season when these winds prevail will be a dry season, as the vapor contained in the air will be further and further from its point of condensation the further south and east it flows.

In our summer, pressure is very low over a great part of the Asiatic continent, owing to the heat and ascending current produced by it; therefore the air of all surrounding regions will flow towards Asia, and the movement will be especially rapid in and near Southern and Eastern Asia, as the greatest oceans of the world, the Indian and the Pacific, approach Asia in this direction.

Pressure is higher on the oceans in summer on account of the comparatively cool temperature which prevails there. Thus the movement of air will be reversed, and the wind in summer will blow from the S. W., S., S. E. and E. This summer monsoon will also be very steady, as the difference of pressure is nearly always in one direction during the whole summer-lower on the land.

Not only is the direction of the movement of air different in summer from that prevailing in winter, the influence on the weather is also different. As the air drawn towards Asia has to pass over a great extent of warm equatorial seas, it is laden with vapor, and this vapor will be deposited in copious showers, especially when it meets a mountain chain, which compels it to rise into higher and cooler regions of the atmosphere. Thus the summer monsoon is the time of cloud and rain for all Eastern and Southern Asia, or the wet monsoon. There is no doubt that the condensation of vapor, giving out its latent heat, is a new and powerful cause for the continuance of the movement in the same direction.

The influence exerted by the heated continent of Asia is so powerful that there is no equatorial calm-belt in the Indian Ocean during our summer, but pressure decreases steadily from about 25° S. L., the polar limit of the S. E. trade, till about 30° N. L. in Northern India, the S. E. trade crossing the equator, and being thus converted into a S. and S. W. wind. On the eastern coast of Asia the tendency of air to flow towards the continent similarly acts on the N. E. trade of the Pacific Ocean, which is drawn in as an E., S. E. or S. wind. We see here the normal or oceanic conditions very seriously disturbed by the influence of the great continental mass, Asia.

I must correct here an error which is frequently made, i. e., limiting the monsoons to the tropical part of Asia, i. e., India, Indo-China, and Southern China. Even on the new Pilot Chart published by the British Admiralty in 1872, this

error exists. It can be easily explained thus: in the tropical seas adjoining India and Southern China, the direction of the wind is N. E. in winter and S. W. in suinmer, and seamen were accustomed to consider as monsoon regions those only where winds of this direction were found. The further north we proceed along the coast of Eastern Asia the more the summer winds become S. E. and E., and the winter winds N. W. and W., yet there is good reason to consider Eastern Asia to the 60° N. L. as belonging to the monsoon region, because here also the winds in winter are from the land; in summer, from the sea, they bring dry, clear weather in the first season, and rain in the second; and last, not least, at both seasons they are very constant. (See Maps, Plates 5, 6.)

For these reasons I consider China, Japan, Mantchooria, the basin of the Amoor River, and the western coast of the sea of Ochotsk, as belonging to the monsoon region.

As to the constancy of the winds I would remark, that the inner regions of India, as, for example, the northwest provinces, Oude, Central India, Punjaub, are generally considered as being situated in the monsoon region, yet the winds are not so constant here as in Japan and the Russian Amoor Provinces.

The continent of Australia may also be considered as belonging to the monsoon region, only the periods are reversed, i. e., our winter is the rainy season there, our summer the dry time. At this season regular S. E. winds are experienced in the northern part of Australia; they may be considered as the S. E. trade, strengthened by the comparatively low temperature and high pressure on the continent. They blow towards the Sunda Sea, and, further on, cross the equator, to appear as the S. W. monsoon on the coast of South China. In our winter, on the contrary, pressure being highest in Asia, and very low in the dry, hot interior of Australia, the N. E. monsoon of China crosses the equator and appears as a N. W. monsoon, bringing clouds and rain to the northern coast of Australia. In these meridians the juxtaposition of the continents of Asia and Australia on the north and south of the line, gives additional strength to the monsoons. Here no equatorial calmbelt is found, neither in our summer nor in our winter, while it exists south of India in the Indian Ocean, as there the monsoons can be said to be single, caused by the Asiatic continent alone, while further east they are double, Asia and Australia both exerting an influence.

It may be asked why the whole Asiatic continent, being equally heated in summer and the air rarefied, does not exhibit monsoons of equal magnitude coming from the Arctic and Atlantic Oceans?

The reason is this: on the Arctic Ocean, pressure is also low in summer, though probably not so low as indicated in Buchan's map of isobars, and besides it is not steady, as on the tropical seas. Yet there is a northern wind coming from the Kara Sea, and blowing through Western Siberia to Central Asia, but it is not as steady as the monsoon of India and China. Besides, as this wind comes from a colder region, it does not bring rain, and thus the secondary influence—condensation of vapor, which is instrumental in producing the monsoons of Eastern and Southern Asia-is not effective here. There flows also a current of air, and a very powerful one, from the Atlantic Ocean towards Central Asia; but, as it

is not from the tropical part of the ocean, it cannot bring much rain and produce the secondary areas of low pressure caused by condensation. Besides, the region of high pressure on the Atlantic is far from the low pressure of Central Asia, and near to that about Iceland; so that the movement in the first direction. cannot be very constant. As to the air from over the lower latitudes of the Atlantic Ocean and the Western Mediterranean, it is attracted towards Africa, which is highly heated in summer, and open to the winds from the surrounding

seas.

The geographical features of the North American continent explain why pressure and winds are so different over it from what is seen in Asia.

The coldest region of America is known to be to the north of the continent, on the islands and ice-bound seas and sounds north of 70°. Ice and snow being bad conductors of heat, the streams of warmer water are thus effectually prevented from having an influence on the air, and the ice-bound seas to the north of America can cool as well as continents.

But, as the coldest space north of the American continent is not separated by mountains and plateaus from the surrounding regions, there cannot be such a constant high pressure there as on the corresponding coldest space of Asia. It will be remembered that the lowest pressure of the northern hemisphere, especially in winter, exists near Iceland, which is partly due to the warm waters of the Gulf Stream. The coldest regions of America are not separated by any natural barrier from this space, and thus air, even from the lowest, heaviest strata, should flow towards Iceland. That this is the case, is shown by the winds in Greenland and on the most northerly stations of the American continent; they are northerly to a very large extent. Probably the easy intercommunication between the coldest region of North America and the region of low pressure near Iceland, explains why the former has not a high mean pressure in winter. Having not a constantly high pressure, the polar regions of America cannot influence the winds in the temperate and tropical regions of this continent as the coldest region of Siberia, with its constantly high pressure, does influence the temperate and tropical regions of Asia. Next, we find a generally high pressure to the south of the United States, on the Gulf of Mexico, as well as on the western highlands and plateaus of the continent, in lat. from 30° to 40° N. Probably, also, pressure is high to about 60° lat. N. on the eastern slope of the Rocky Mountains, where, the winter being cold, the Rocky Mountains in the west not permitting the air in the lower strata to flow towards the Pacific, and the depression about Iceland being far away, there exist all conditions for a high pressure. But barometrical observations from this region are wanting.

Thus, the Mississippi Valley and seaboard of the United States have in winter regions of high pressure to the S. and W. of them; i. e. they are exposed to the influence of winds from different directions, of which those that come from the S. are warm and laden with vapor, and thus able to sustain the precipitations necessary to the progress of storm-centres, while the air from the W. and N. W. is cold and dry.

A country generally level, subjected to such different influences, must have a

very variable climate, and this is known to be the case in the United States. Nowhere in the same latitudes are the variations of temperature and pressure so great and sudden as in the Mississippi Valley and in Texas. On the Atlantic sea-board the variation is somewhat less, owing to the slight protection afforded by the Appalachian Chain.

In summer again, there are no parts of North America which are as strongly heated as the interior parts of Asia, none also where pressure is as low, and thus there are no monsoons comparable in strength and constancy to the summer monsoons of Asia. Especially is this the case with the eastern part of the United States, where the land is so much pervaded by the influence of the sea that there is scarcely a summer depression of the barometer. The Gulf of Mexico is situated just in the latitudes where pressure would be lowest on a great continent, and, owing to the relative coolness of the air over great bodies of water, pressure is nearly as high over the Gulf in summer as in winter. Yet, as there is a rarefaction of the air in the interior and western part of North America, there is a monsoon wind drawn in from the Gulf of Mexico to supply the deficiency. The mean direction of the wind is southerly in summer over a great part of the United States east of the Rocky Mountains. It is more S. E. in Texas, and S. and even S. W. in the States north and northeast of it, partly due to the earth's rotation, and partly also to the influence of the lower pressure in the lake regions on the air over the Gulf of Mexico. On the Atlantic coast the winds have some monsoon features (as was shown by Prof. Coffin in 1848) but still the flow of air is much more from the southwest than would be the case in a real monsoon region, the ocean being to the east.

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If, aside from disturbing influences, we consider only the mean direction of the wind, the influence of the Gulf of Mexico is seen to be paramount over a large and important region of the United States, extending from the Mississippi to the Appalachian Chain and from 31° to 42° N. L. The mean direction of the wind is about W. S. W. at all seasons, with a ratio of resultant of about 30. The cause of this is, that pressure is highest at all seasons to the S. and lowest to the N. and N. E.

Having now considered the influence of the pressure of the air on the direction of the winds, the influence on force remains to be shown.

It is easy to conceive, that, the influence of pressure once acknowledged, this

influence would be the greater, the nearer areas of high pressure approach areas

of low pressure, or, in other words, the nearer any given difference of pressure was found to exist. It was to be supposed, that the more this was the case, the greater would be the velocity of the winds. This has been found to be really the case.

This difference of pressure relative to distance was called by Stevenson barometric gradient. This term of barometric gradient may be applied to the mean direction of the wind, and the rate of progress, as well as to any given single observation. The more the isobars are crowded together, the steeper is the gradient, and the greater will be the velocity of the wind, all other conditions being the

same.

There are conditions well known to science in a general way, although not

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measured with accuracy, which prevent all winds from reaching the same velocity even if the relative distance of the isobars be the same.

These conditions must be considered in brief.

In the lowest stratum the velocity is lessened on account of friction on the surface of the earth, while the higher are also more or less affected by the friction of the different strata on each other.

The winds on the ocean will be less affected in this way, because of the smooth surface of the water. The greater velocity of the wind on the sea is well known. The figures published in the "Quarterly Weather Reports" of the Meteorological Office, of London, very clearly show the decrease of velocity in the interior of Great Britain even in level parts of the country.

The following table shows this for the United States. I give the mean velocity of the wind in a group of inland stations (Eastern New York) compared with that of the sea-coast (Cape Cod and adjacent islands) and also with the summit of Mount Washington, the highest peak of the New England States.

MEAN VELOCITY OF THE WIND. MILES PER HOUR.

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Eastern New York
Mount Washington'.
Cape Cod and islands

2.4 5.1 5.7 5.4 4.5 5.1 5.8 4.8 3.3 10.4 7.9 5.7 8.7 7.5

...

|17.4 21.0|17.3|15.5| 24.3||50.2 41.7 36.8 38.8 41.8 34.0 44.8 52.2|

7.8 10.9 5.3 9.0 6.7 9.6 9.3 6.3 19.9 20.5 12.2 16.1 10.6 10.9 10.9 20.0

Mount Washington having the freest position, the strength of the winds there must be considered as more nearly normal than at the other places. The N. W. winds are the strongest, both summer and winter. But in the vicinity of Cape Cod, the N. E. winds coming over the smoother surface of the sea, are the strongest.

It is safe to present the following rules for the velocity of the wind. It is greater: 1. On high isolated peaks, than at low stations.

2. On the seashore, and especially on isolated islands, than in the interior of continents.

3. In level countries than in countries surrounded by mountains.

4. In prairies, and especially desert countries, than in wooded regions. These rules apply to the local positions only. But we may remark that it is possible to mention some regions where the velocity of the winds is greater, others where it is less, than the average over the whole earth.

To the latter belong the equatorial calm-belt, and the calm-belts at the polar limits of the trade-winds. It would be wrong to imagine that any point on the

1

85

One summer and two winters, 1870–71, and January, February, and December, 1872.

July, 1875.

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