LETTER LIV. The Winds. MY ESTEEMED Children, The universal pressure of the atmosphere necessarily occasions every part to seek an equilibrium. If the elasticity from any cause is diminished in any part, the surrounding parts rush towards the vacuum, and this rush constitutes a current of the mass, or a wind, which begins at the part and extends to great distances. Thus heat increases the volume or mass by day, and the cold of night reduces it again, creating a vacuum, and the breezes of evening, or the flowing in of air from surrounding parts. This is peculiarly the case between the tropics, where, in consequence of the regular heat from the passage of the sun, a current always follows it from East to West, and generates the Trade Winds. North and South of the tropics, the winds being under no paramount influence, like that of the sun, are created by different circumstances, and variable as their causes. In Great Britain they blow chiefly from the Atlantic, and then westerly winds prevail two-thirds of the year, bringing with them the clouds created by the vapours of the sea, and watering and fructifying our beautiful country. The east winds, on the contrary, come from the high lands and deserts of Siberia, and are cold, dry, and ungenial. The north winds from the Icy Seas are proverbially cold, and the south winds, which come from Africa across France, are bland and genial. In North America, the east and west winds have the opposite character to ours, for their east wind brings rain from the Atlantic, and their west wind cold and catarrhs from the woods and lakes. In the East Indies they have periodical changes in the direction of the winds, called Monsoons, arising partly from the sun and partly from the Asiatic mountains, which, north of Hindoostan, form the highest ridges in the world. The wind, therefore, blows half the year one way and half the other, and the period of change is marked by destructive hurricanes. Whirlwinds are caused by the meeting of currents of wind, and they sometimes make a vacuum or species of hole in the air, like a vortex in water, so that the clouds descend in them, and water and other bodies are lifted from the ground, forming water-spouts, and conveying ponds of water with frogs and fish to a distance. The fury of the winds would seem to be confined to the sea, but it is not so. Land winds are capricious and violent. They rush with irresistible fury. They excite impetuous commotions. They not only raise mountains of sand in Arabia and Africa, and overwhelm large plains, but they carry sands into the sea, and form banks and even islands. It has been observed, that no cloudy and moonless night, which is very windy, is dark; as though the wind by its motion always raised some degree of light through the agitation of the elements, and the collision of the particles that are floating in the air. Hurricanes are the scourge of the West Indies, of Madagascar, and other countries. They sweep away trees, plants, cattle, and the soil that nourishes them. They drive back, they annihilate, rivers, and produce new ones. They overthrow rocks and mountains. They scoop out holes and gulfs in the earth, and totally change the face of those unhappy countries. According to computation a common high wind travels at the rate of thirty-five miles an hour. A balloon has been carried by a gale of wind 120 miles in two hours; but an extraordinary gust of wind in England, in 1703, was calculated to have had a velocity of eighty miles in the hour. Its general cur rent, however, in a storm, may be estimated at from sixty to seventy miles an hour. Many instances may be adduced of the pestiferous and fatal changes which the atmosphere partially undergoes, especially in Africa and Asia. The si rocco, and other hot winds, sometimes prevalent in the southern parts of Europe, are well known. The Samyal wind, which prevails for certain days in the year, not only in Egypt, but in Arabia, Syria, Persia, and India, and even in Africa and Spain, is so disasterous in its effects, that every thing in nature seems to yield to its power. Man and animals immediately fall victims, if they are surprised by it in its fury. This wind, however, and those similar to it, it is curious to know, always proceed from desert countries. It is the action of the sun on sand, not tempered by the intervention of forests, rivers, or lakes, which renders their heat and aridity so destructive. The winds are, in various ways, not only salubribrious but most beneficial to man. Their continual vicissitudes give a turn to the dispositions and productions of nature; they correct those ill consequences which would unavoidably flow from a tedious inactivity and state of rest; and, in short, they invigorate and purify the air, by keeping it in perpetual motion. Without them, our habitations would become unwholesome, and great cities would in a short time be as nauseous as common sewers: in a word, all nature without winds, and the changes which they introduce, would languish and die away. **** LETTER LV. Heat-Light-Fire-Oxygen-Nitrogen-Hydrogen- MY ESTEEMED Children, I have already remarked on the circumstance that the atmosphere in which we live and breathe consists of two kinds of atoms, which, taken separately, constitute the two principles, called acid and alkaline, but more commonly Oxygen and Azote, or Nitrogen. Their union produces the neutral and tasteless body of air, called atmospheric, and their proportions are nearly 1 or 21 of oxygen to 4 or 79 of azote or nitrogen. An excess of either is not beneficial to animal bodies; but an excess of azote is very injurious, for no animal can live in azotic gas. But as various bodies disturb the equilibrium or due proportions of the two, the economy of nature tends in various ways to correct the inequality. I have also explained that the air and these two principles necessarily consist of atoms exceedingly small, but yet different enough to cause their different effects. They are, however, so small that man can never behold their action, and determine the exact cause of the differences. The gaseous state of such atoms, or their power of filling a space as completely as though they were solid gold, can only arise from their being in great motion; while from mutual collision these motions must necessarily be circular, and hence their power of sustaining animal life, by being partly fixed in the lungs during respiration, and their power of sustaining combustion by being fixed or parting with their motion during the process. We need not wonder at our vigour, strength, and motions, if we fix in the lungs a million of moving atoms every time we breathe; nor at the heat of a candle, if in every second it does the same. fixing of the oxygen gas displays or concentrates heat in both cases. The To restore the consumed oxygen, nature, or the supreme wisdom of the God of nature, has employed plants, and these correct what the other vitiates for they flourish best in air vitiated by breathing. The powers or economy of plants is chiefly exerted in the sun-shine, because excited by the action of light, hence they absorb oxygen by night, and do not evolve it again till the day, when the sun-shine restores their energies. From this cause the night-air is less invigorating to animals, than that of day, while it is loaded with the vapours which fall after the departure of the sun. In like manner, it is pernicious to sleep in a close room in which flowers or plants are placed, for they absorb the oxygen or vital part of the air, and leave only the azote, or part in which alone animals cannot live. They act just like burning charcoal, which in like manner absorbs the oxygen, leaving all the azote, and this destroys respiration, or renders respiration useless. Whatever consumes the proportion of oxygen which is necessary to constitute good air, that is, one-fifth of the mass, renders air unfit for the purposes of sustaining animal life. All combustion does this, whether of charcoal, or coals, or candles, for combustion or burning is merely the union of oxygen with the inflammable body. Common fires are less pernicious merely because as they rarify the air and cause a current up the chimney, a fresh body of air comes into the room through the crevices in the doors and windows. Charcoal fires in chafing-dishes and earthen-pots, and vegetables and flowers, are dangerous, because they create no current from without, but having consumed or fixed the oxygen of the air, so useful in respiration, the azote or nitrogen is not restored to its due pro-portion by any accessions of that mixture which constitutes pure air, like brandy and water. Other accidents of air merit notice Thus the atoms of carbon, so combine with oxygen as to form a gas one and a half times heavier than atmospheric air. Hence it sinks like water to the bottom of empty wells and unventilated places, and there puts out lights or chokes those who get immersed in it. Besides this, there is another gas, sixteen times |