11.] AIR COOLED BY ASCENDING. 29 To answer this, we will revert to the familiar experience afforded by our fire. When it is in full blaze, the smoke (which we now know to be carried by the heated air) rises in a rapid stream. But it does not rise very high; perhaps as high as the tops of the palm trees, and it then spreads out and remains for a time suspended in mid air. It ceases to rise because it has already lost a great deal of the heat that caused it to ascend, and has become no lighter than the air around it. So it is with every current of air that ascends, whether it is set in motion by the heat of a fire or by that of the sun. Some very important experiments made by Mr. Joule have enabled us to ascertain exactly the amount of cooling that air undergoes under these circumstances. It is such that if a quantity of air, as hot as it usually is at Calcutta about nine in the morning in May, were to rise from the ground, it would be cooled to the temperature of ice by ascending 9,100 feet or to about 2,000 feet above Darjiling. In ascending mountains, the higher we go the colder does the air become ; but not so rapidly as a mass of air that is in motion upwards. At Darjiling, for instance, the warmth of the air (called its temperature) is about half-way between that of the air on the plains, and that of melting ice; and we should have to ascend to more than twice the height of Darjiling, before we should find the snow in the summer time lying unmelted on the slopes of the mountains. It is owing to the expansion that the air undergoes during its ascent, or rather to the resistance which it overcomes in thus expanding, that it is cooled. I We have now found the explanation we sought. The highly heated air over the plains of India ascends and cools as it ascends. Air highly charged with vapour is drawn from the surface of the seas to the southward, and this, ascending in its turn, is cooled and forms dense clouds and rain. When it has risen to a great height and has got rid of most of its vapour, it turns and flows back to the south, where descending again to the sea-level, it takes up more vapour and then repeats the same circuit. 30 THE MONSOONS AND TRADE-WINDS. [CHAP. On the other hand, in December and January, when the sun is low and the surface of the land is colder than the sea; and when Australia and places south of the Equator are most heated by the sun, the air flows from us towards this heated region, in the lower atmosphere, and returns to us as a wind in the upper atmosphere. The lower current is our winter or north-east monsoon. It is so called, because at sea it really blows from the north-east, but in Bengal and the North-West Provinces it comes chiefly from the northwest or north. The upper return current, which may be called the Anti-monsoon, is felt on the Himálaya, and descends in the North-West Provinces and the Punjáb; and the vapour it brings furnishes their winter rains, on which the rabi crops depend. In India, then, we have the wind blowing one way during four or five months of the year and in the opposite direction during another four or five months; all such winds are called monsoons. In the China Sea and Australia, as well as in one or two other parts of the tropics, monsoons are felt much like ours; except that the directions are not the same, since they vary with the form of the land and the direction of the coast-line, and they vary also accordingly as the land is situated in the Northern or in the Southern Hemisphere. The tropical sea, which furnishes the vapour to the summer monsoon of India and China, lies to the south and southwest, while it lies to the north and north-west of Australia. While, therefore, our south-west monsoon is rainy and our north-east monsoon is a dry wind, the north-west monsoon of North-West Australia is wet and that from the South East is dry. In the middle of the Atlantic and in the Pacific and South Indian oceans, the wind blows the same way all the year round, viz., towards the Equator from the tropics, and these are called trade-winds. To the north of the Equator the trade blows from the north-east, and to the south of it from the south-east, while between the two, and not very far from the Equator, is a well-defined belt where the air is generally calm, and II.] ANTI-TRADES. CAUSE OF THE SEASONS. 31 which is therefore termed the belt of calms, and in the Atlantic Ocean, the doldrums. Over this belt, the air which pours in from both sides, rises to the higher parts of the atmosphere, and then flows away in two directions opposite to its former course, viz. towards the north-east in the Northern and towards the south-east in the Southern Hemisphere. These return currents are called the Anti-trades. At the place of their origin over the belt of calms they are at a great height, higher than the tops of the loftiest mountains, but they descend gradually to the earth, becoming more westerly as they advance, until in Europe, Northern Asia, and North America on the one hand, and over the Southern Ocean on the other, they prevail as more or less westerly winds. In the Northern Hemisphere, however, they are very irregular, and alternate with winds from the east and north-east frequently and without much warning, so that the expression "as changeable as the wind" has there become proverbial: Nevertheless, this irregularity is only apparent and not real. There are in Europe, as in India, two principal winds, one of which blows from the cold northern countries towards the Equator (where it is hottest); the other, the Anti-trade, which begins near the Equator and blows thence towards the north and north-east. It is this last that brings Europe most of its rain, just as our anti-monsoon brings winter rains to the Punjab. The regular change of the monsoons, and the alternation of summer and winter, of wet and dry, or hot and cold seasons, with which we are all familiar, depend on the peculiar position of our earth relatively to the sun. They depend on the direction of the earth's axis, that imaginary line around which the earth rotates, as I have already explained in the previous chapter. In the month of June the positions of the earth and sun are such as are shown in Fig. 2, in the month of December as shown in Fig. 3. To trace the consequences of these two conditions, let us select two points on the earth, marked a and b in the figures, the one in the Northern, the other in the Southern hemisphere, 32 CAUSE OF THE SEASONS. [CHAP and let us see how they will be affected respectively in June and December by the sun's heat and light. From what has been said in the previous chapter, it will be easily seen that the shaded half of the earth in both figures represents the dark or night side and the unshaded half the light or day side. As the earth revolves, the two selected points a and b will be carried round the axis in the direction shown by the dotted lines a a1 bb1. Now in Fig. 2, representing the June position of the earth, the greater part of a a1 is in daylight and only the shorter part in darkness. This shows why, in June, a place in the position a has a long day and a short night, the difference being the greater the farther north the place is situated. On the other hand b in the Southern Hemisphere has a short day and a long night. The one is receiving the Direction of sun's axis II.] CAUSE OF THE SEASONS. 33 sun's heat during considerably more than twelve hours, and is cooling during considerably less; the other is in the reverse case. For this reason alone a would be much warmer than b, and would enjoy summer while b is in the depth of winter. But this is not all. On a the sun's rays fall directly, or almost so, at noonday, while on b they fall very obliquely, explaining the familiar fact that in summer the sun is high up in the heavens at noon, at Calcutta quite overhead, while in winter it is comparatively low at the same time of day. That the heat of the sun is the greater, the higher it is in the heavens, is so familiar to us all that we might almost dispense with further explanation. But the figure serves to explain this fact also. The zone of the earth between a and m, in Fig 2, is as broad as the zone between band n in the opposite hemisphere, but while the quantity of the sun's heat and light which falls on the former is all that which passes between the arrows marked and s, the quantity received by the latter is so much only as passes between the arrows and u, which is less than one-fourth of the former quantity. Hence the month of June brings summer to a and to all other places in the Northern Hemisphere, while at b and all places in the Southern Hemisphere this is the season of mid-winter. In Fig. 3, which exhibits the December position of the earth, the conditions of a and b are reversed. a has a short day and a long night, and even at noon receives but little heat comparatively, since the sun is low and its rays fall in a very slanting direction; while b enjoys long days and the more direct rays of the sun. The former, together with all other places in the Northern Hemisphere, has now midwinter, the latter together with the whole of the Southern Hemisphere midsummer. On the equator, which is the boundary line between the two hemispheres, the figures show that the day and night are exactly equal both in June and December, and the same is there the case all through the year. On the other hand, at the two poles, there is but one day and one night in the year. The North Pole has daylight from the 20th March B.G. D |