WINTER is gone, but summer

is not come; and now, in the middle of what ought to be spring, winter seems come back to us. This little confusion of the seasons does not surprise us; in truth, we expected it; and we only say 'the east winds have begun.' They are annual visitors; and we set ourselves with stony fortitude to bear the infliction of their presence, for we know they will stay with us till deep into May. Meantime, we are given over to unmitigated wretchedness. A dead, dull canopy of clouds is overhead, or a glaring sun in a whitey-blue sky mocks our misery; but always the same sweep of the relentless wind-cold, dry, harsh, pinching the flesh, searching the bones, withering the spirit. There are some persons— Mr. Kingsley is one of them-who say they like these east winds, and who write odes to them. What a singular idiosyncrasy! Can any

man be honestly poetical with grit in his whiskers, small pebbles in his eyes, a blue nose, and a shiver all over? Is a shout of jubilation under such circumstances a desperate attempt at self-deception, or is it a deliberate imposition on the public?

Of

Nevertheless, unpleasant as they are, these east winds owe their ultimate origin to the same physical motive power as that which gives birth to the genial zephyrs. late years, much has been done in the investigation of the sources and laws of the winds, and many curious facts touching the currents of air and ocean all over the world have been established.*

We are still very far from complete knowledge of such phenomena, but the knowledge we have acquired has proved already of the utmost service to the navigator. By directing him to the most favourable tracks, it has been the means of greatly shortening the time occupied in his voyages, diminishing it in some

instances by fully a third. I fear that science has not yet arrived at the discovery of any means of shortening our east-wind season. But while we are undergoing it, we may at least have the grim satisfaction of knowing why it comes, and why it must of necessity cease a few weeks hence. To find out these reasons we shall have to wander all round the globe; and so we may in imagination bask in sunny lands, and bathe in tepid seas, while our bodily presence is cowering over a sea-coal fire.

Seeking for causes for our east winds, the first question that suggests itself is, Why should there be in any place, wind in any direction? Our sea-coal fire shall yield us answer. Heat expands air, as it does everything else. Expanded air is lighter, bulk for bulk, than. colder air. Lighter, it tends to rise above colder and heavier air. So the smoke goes up the chimney. Smoke is not in itself lighter than the surrounding air-it is, on the contrary, heavier-and when it has had time to cool it will, if not carried away by wind, fall down again in smuts and yellow fog. It rises only because it is, for the moment, hotter. As it rises, cold air comes in through the chinks of the door and windows, and flows along the floor to supply the place of what has gone up the chimney. It is warmed as it approaches the fireplace; it becomes very hot as it passes through the fire; it is sent upwards; and so a constant draught along the floor is kept up. We can feel it with our hands if we put them down near the floor. It is but slight if the fire is small and the room large, because then the current is small in amount and has a large space from which to draw supplies. But a large fire in a small room makes a very distinct current along the floor. This current is wind.

Now on the globe the equator is

nearly the line of average greatest heat. It therefore corresponds to the fire-place in a room. The air is heated by contact with the warm ocean and burning plains of the torrid zones, and therefore, by the law of its nature, it rises. To supply its place, colder air from the temperate regions flows in. We should therefore expect to find, within the tropics, winds blowing along the surface of the earth directly towards the equator, and meeting there. And so we should find them, if the earth stood still. But it revolves on its axis once every twenty-four hours. The speed of this revolution is greatest at the equator, because there the circle which revolves is larger than any other parallel circle nearer the poles. Thus a point on the equator travels at the rate of about one thousand miles per hour, but a point on the tropics travels at the rate of only about nine hundred miles per hour. In passing from the tropics to the equator, therefore, air which was apparently not moving at the tropics, because really travelling at the same rate as the earth's surface, would find itself moving one hundred miles per hour less rapidly than the earth's surface at the equator. It would appear, therefore, to be blowing a hurricane in a direction opposite to that of the earth's revolution. This revolution is from west to east; and the wind would thus seem to blow from east to west. As in reality, however, it is undergoing the change gradually during the whole time it is passing from the tropics to the equator, it ought only to blow moderately from an easterly direction; at the same time that in the north hemisphere it is blowing from the north, and in the south hemisphere it is blowing from the south. In the northern tropical regions, therefore, we should expect to find a north-east wind, in the southern tropical regions a south-east wind.

This is exactly what we do find. These north-east and south-east winds are the 'trade winds' of the tropics; so called, because of their constancy being of such advantage to the merchant adventurer.

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They blow steadily and uniformly throughout the whole year in the precise directions which our theory has indicated. At their place of meeting, near the equator, there is a region, varying in breadth from a few miles to five or six degrees of latitude, of perpetual calms. These calms are accompanied with incessant rains. Sometimes the fresh water may be scooped up from the surface of the sea, so heavy are the rains, and so still the winds. In these rains we have another proof that on this line the air rises. the laws of nature (I must here simply state them, without adduction of any proof) ordain that, as air rises, it loses heat, and that cool air cannot hold as much aqueous vapour as warm air can. Hence the surplus aqueous vapour which it can no longer hold, is precipitated; it forms clouds, and falls as rain.

For

As there is no accumulation of air at the equator, in spite of the winds which constantly blow towards it, it is evident that the air must return to the north and south as upper currents. But as it has gained, while at the equator, the same velocity as the earth, so as seemingly to be in a dead calm, it must, when passing more slowly moving higher latitudes, appear to blow in the direction in which the earth is moving. These return currents, therefore, will seem to us to blow from the south-west, as in the southern hemisphere they will seem to blow from the north-west. Thus all over the earth we have, as a general rule, two sets of winds in the upper and lower strata of the atmosphere-the one from the poles to the equator, with an apparent slant from the east; the other from the equator to the poles, with an apparent slant from the west. But while, within the tropics, the winds which blow towards the equator are those which are next the surface of the globe, the case is otherwise in the temperate zones. For at the tropics there is an interchange of position between the lower and upper currents of air; the winds which blow from the equator there descend and pursue their

course towards the poles as the under-currents, sensible to us as wind. Thus, then, in the northern hemisphere the south-west, in the southern hemisphere the northwest, become the prevailing winds, and the north-east and south-east become for the most part the upper currents of the atmosphere. For this interchange philosophers have not yet discovered the reason; but we all may at least appreciate the advantages of the arrangement. It gives within the tropics, in the shape of the trade winds, the cool polar breezes, unheated by intermediate contact with the warm earth. It gives in the temperate regions the warm equatorial breezes, laden with the vapours which they have sucked up over the tropical seas, to be distilled on us in the shape of the gentle showers which our west winds bring. Had the system been reversed we should have had winds constantly dry but piercingly cold in winter, and burning hot in summer. Our easterly winds in spring, coming in the mean of temperature, give us a taste of the parching drought which we should have had all the year through had the polar breezes been the common surface currents, instead of being generally transmitted on their course as upper currents high over our heads.

Still, however, accepting the fact that the westerly winds are those which for some reason or other are ordained to prevail with us (as they do in the proportion of about three to one), we have yet to ask why this rule should be subject to a periodical reversal in spring. In other words, why should the polar winds blow then, while at other seasons the equatorial winds chiefly blow? Now, it happens that in other parts of the earth than ours there are periodical shifts of the prevailing wind. Where these divide the year between them, they are called monsoons. They are not always, however, of equal duration. One monsoon may continue for eight months, while the other lasts only two months. More frequently they are each of about five months' continuance, while for a month at

each period of change the wind is variable. Let us now see what are the causes that occasion these monsoons, for perhaps in them we may find an explanation of our short easterly monsoon.

The sun shines equally on sea and land. Sea and land, therefore, receive an equal amount of heat. But they dispose of it differently. The land receives it only on the surface, and keeps it near the surface. While the surface is burning beneath the summer glare, a very little way down-in a shallow well, for instance, or even a cellar-it is comparatively cool. But in the clear sea the sun's rays penetrate many fathoms below the surface, and the upper layer of water therefore takes but a slight passing toll of his heat. Moreover, the waters are in constant agitation. They are stirred by winds, and they are borne away and renewed by currents, of which I shall hereafter have more to say. Besides all this, water holds a great deal more heat than dry earth does, although to the senses, or even the thermometer, it is not higher in temperature. This is a very singular and very important law of nature. Certain

bodies absorb a great deal more heat than others when they are raised to the same sensible temperature. Water, for instance, must be exposed to a lamp thirty-three times as long as the same weight of mercury, in order to bring it to the same heat by the thermometer. It must be exposed four times as long as the same weight of air, and nearly three thousand times as long as the same bulk of air. The heat which it thus absorbs and makes to disappear is not, however, lost. In cooling it is all given out again; and thus water in cooling gives out as much heat as thirty-three times its weight of mercury, or four times its weight of air. So it bottles up the greater part of the heat which it receives from the sun; it does not rise much by the thermometer, but it holds the heat, and will take a long time before it cools down.

From the combined effect of all these causes the surface of the land is heated in summer much faster,

and to a much higher degree, than the surface of the neighbouring seas. So wherever we have land, we have in summer a fresh case of the fireplace in a room. The hot soil warms the superincumbent air; it rises, and draws in the air all round to supply its place. Hence, upon a small scale, we have upon islands or coasts the alternation, by day and night, of sea and land breezes. By day the land grows hotter, the air over it rises, and the cool seabreeze sets in-shore. By night the land cools faster than the sea; the sea is now the warmer, the air over it rises and draws the cool landbreeze from the shore. On a large scale the process is the same with the alternation of the seasons. The great continents become in summer hotter than the neighbouring oceans, and draw in the air from the ocean. In winter the continents become colder than the sea; the sea may perhaps draw air from the land, or at least the land will cease to interfere with the great system of aërial currents. These winds thus changing with the seasons make the monsoons of southern lands. In the East Indies, for example, during the winter months the north-east monsoon prevails. It is really the northeast tradewind, which there is then no disturbing cause to modify. But in spring the sun begins to heat the great arid plains of Central Asia. From the points nearest to that influence air begins to flow to it. As the season advances the influence grows more importunate, and a larger surrounding area yields to its demands. Thus the regular north-east trade-winds are stopped, and a counter current from the south-west is set up. This constitutes the south-west_monsoon, which in the north of India commences in February and March, but in the south not till April or May. When, in September and October, the Asian continent begins again to cool, the south-west monsoon ceases, and after a period of variable winds the regular tradewinds or north-east monsoon resumes sway.

Exactly a similar process takes

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place with all the other great continents. During their summer, when the sun is beating hotly upon them, there is a sucking in of air from all sides. During their winter they cease to disturb the general flow of the atmospheric currents. It happens that the continent to which we are nearest lies in the direction of the prevailing currents. In summer therefore we have no disturbing, but rather an intensifying element at work. Our regular westerly winds consequently blow in summer with increased regularity. They may be a little to the north of west or a little to the south of west, according as clouds and sunshine play over the broad face of Europe. But from June to September there are very few days in which the wind is not westerly. This fact gives, however, as yet no reason why a season of easterly wind should precede. For the cause of such an exception to the rule we must evidently look in another direction. It must be the Atlantic Ocean, and not the continent of Europe, which exerts the power that thus draws the wind back and reverses its natural course at the very moment when we should expect it to become stronger and steadier in that course. There must be at this season some peculiarity in the Atlantic Ocean and in the narrow seas by which we are surrounded to give it this abnormal direction.

Such a peculiarity we shall in fact find in the course of the great ocean currents. In all the oceans there are such currents, whose flow is as steady and definite as that of a river. These ocean currents arise mainly from the same cause that sets the wind in motion. The water in the torrid zones becomes hotter, it expands, and flows away over the surface, while colder water from temperate and polar zones flows in beneath to supply its place. As water is less mobile than air, and as we have seen that it grows hot and cold much more slowly than air, these ocean currents are far more regular and steady than even the trade-winds themselves. One of the most powerful currents, as well

as one with which we are best acquainted, exists in the North Atlantic. It is known as the Gulf stream, from apparently taking its rise in the Gulf of Mexico. But before reaching the Gulf it has crossed the Atlantic from the coast of Africa, drawing its supplies from a system of corresponding currents partly in the northern and partly in the southern hemisphere. All the way it has been exposed to the heat of the tropical sun. In the Caribbean Sea and the Gulf of Mexico its course is contracted, while the perpendicular rays still beat fiercely upon it. When it

issues then from the Gulf of Mexico by the only free exit, the channel between Florida and Cuba, it has a rate of nearly five miles an hour. At the narrowest part of the straits it is about thirty-five miles wide, and it is several thousand feet in depth. From this point it flows to the northward, alongside of, but not touching, the American coast, till it reaches the latitude of Philadelphia. Here its breadth has increased to nearly a hundred miles, while its rate is still about three miles per hour. Its depth has consequently considerably diminished, but it is still not less than three thousand feet. From this point it strikes across the Atlantic, spreading more upon the surface and with a diminished velocity as it advances. In September the flow is most rapid and its deflection the least. then takes a north-easterly direction from the banks of Newfoundland, and passes in the clear way between Great Britain and Iceland. In March it moves most slowly, and takes a course nearly due east till it approaches the south of Europe. Here it divides, a part turning southwards, while the main body flows northwards; skirting, though here also not quite touching, the coasts of France, passing near, and partly through the channels of the British Islands, and pushing on near the Norwegian coast. Between it and this coast there is always a counter current or back draught of cooler water. From March to September, and from September to March, the Gulf

It

stream oscillates between these extreme limits.

On

The existence of the Gulf stream was first made known by Franklin. Its direction and force have been recently ascertained by scientific observations, but they have long been popularly known by the driftwood of strange countries which the stream casts upon our shores. It is common to find trees and plants from the West Indies thus thrown upon the rocks of the Orkney and Shetland Islands. But a still more remarkable test is furnished by the thermometer. In the Gulf of Mexico the summer temperature of the sea is 86°. Off New York the middle of the Gulf stream has still a temperature of 80°; while the water between it and the shore is not above 60°. In winter the difference of temperature is still more striking. each side the waters are scarcely above the freezing-point, while in the stream they are 60° or 70°. Even when it reaches our shores, so slowly does water cool that the winter temperature is still scarcely under 50°. The returns of the Scottish Meteorological Society (it is much to be desired that similar observations on the temperature of the sea were made by English and Irish meteorologists) inform us that the average sea temperature off the Shetland Islands in January is 48°, or as high as the average winter temperature of Rome. Far into the Polar regions can we trace the penetrating influence of this benign warmth. It keeps the seas of the North Cape perennially open; while on the American shore, Canada and Newfoundland, in the same latitude as Bordeaux, are fast locked in ice.

To supply the place of the waters brought by this majestic flow from the equator, there must necessarily be counter-currents from the Arctic regions. The principal of these descends along the east coast of Greenland. To the south of Greenland it joins another current which descends Davis' Straits. It was this current that brought the Resolute, which was frozen in and abandoned by her crew in Melville