Our planet Earth is surrounded by a blanket of air called atmosphere. As the air has pressure , so it exerts pressure , which can be measured. The instrument used to measure atmospheric pressure is called barometer.We need to know the variation in atmospheric pressure to forecast the weather. It is also used to measure the altitude or height of objects above the sea level. In 1943 Evengelista Torricelli, an italian demonstrated that air exerts pressure at sea level is normally 76cm of mercury column . He made a simple device mercury barometer for measuring atmospheric pressure. A simple barometer consists of a glass tube, open at one end and closed at the other end. It is filled with the mercury. When turned upside down, and immersed in a container of mercury in process , there is a drop in its level in the tube. Rest of the mercury is held up in the tube due to the air pressure.The height of the mercury column in the tube indicates the atmospheric pressure. This simple device was later improved by a scientist called Fortin.Now a days, two types of barometers are used to measure the atmospheric pressure. These are mercury barometer and aneroid barometer . In a mercury barometer, there is a printed scale beside the tube containing mercury. It gives the measurement of pressure. They are heavy and are difficult to move from place to place. Aneroid barometer doesnot use any liquid. It consists of a cylindrical box which is exhausted of air and closed with a thin elastic diaphragm. The diaphragm moves in and out with the variations in the atmospheric pressure. The change is indicated by a pointer which moves on a scale by a system of levers. Aneroid barometers are smaller in size than mercury barometers and thus are easier to carry. Now atmospheric pressure is measured in the unit "millibar". One millibar is equal to a column of mercury - 80 cm high in a tube of 1 cm diameter. Earlier, it was measured in the unit " torr". One torr is equal to 1 mm column of mercury.
2)Northern Lights : 
Northern Lights are long waving streamers of light, often seen in the night sky in the northern hemisphere during both cold and warm weather . These wavering illuminations phenomenon are also called the Aurora Borealis, from the Latin words meaning a " northern dawn". They are most frequently seen between 65 degrees and 80 degrees northern latitude, but the area of visibility extends further South in the North America than in Europe. The aurora of the Southern Hemisphere is called the Aurora Australis, from the Latin for " southern dawn".The bands of light in the aurora seem to radiate from an arc and send their rays far across skies.They are most often white , but are sometimes green , red or yellowish. The rays sometimes are straight, may wind backwards and forward, sometimes they look like afar or form a crown round a dark center. Their movement is so rapid sometimes they are also called the " Merry dancers".Scientific studies of this natural phenomenon began in 1716 when there was a spectacular display all over European skies. The English Astronomer Edmund Halley (1656-1742) proved a connection between them and the earth's magnetism. But the exact cause and physics of the auroras is still not completely understood. The most likely theory is that they have their origin in streams of electrically charged particles from the sun, which are turned aside to the north and south magnetic poles on reaching the upper layers of the earth's atmosphere. Auroras are the most pronounced during magnetic stroms, that is during the time that the earth's magnetic field is most disturbed. They also tend to occur when there have been signs of unusual activity in the sun.
Northern Lights are long waving streamers of light, often seen in the night sky in the northern hemisphere during both cold and warm weather . These wavering illuminations phenomenon are also called the Aurora Borealis, from the Latin words meaning a " northern dawn". They are most frequently seen between 65 degrees and 80 degrees northern latitude, but the area of visibility extends further South in the North America than in Europe. The aurora of the Southern Hemisphere is called the Aurora Australis, from the Latin for " southern dawn".The bands of light in the aurora seem to radiate from an arc and send their rays far across skies.They are most often white , but are sometimes green , red or yellowish. The rays sometimes are straight, may wind backwards and forward, sometimes they look like afar or form a crown round a dark center. Their movement is so rapid sometimes they are also called the " Merry dancers".Scientific studies of this natural phenomenon began in 1716 when there was a spectacular display all over European skies. The English Astronomer Edmund Halley (1656-1742) proved a connection between them and the earth's magnetism. But the exact cause and physics of the auroras is still not completely understood. The most likely theory is that they have their origin in streams of electrically charged particles from the sun, which are turned aside to the north and south magnetic poles on reaching the upper layers of the earth's atmosphere. Auroras are the most pronounced during magnetic stroms, that is during the time that the earth's magnetic field is most disturbed. They also tend to occur when there have been signs of unusual activity in the sun.
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