【正文】 ved the clouds /over islands. There is a special cloud formation /which indicates /there is land/ close by. Using wildlife Seaweed Sailors often saw seaweed/ in the sea /and could tell /by the colour /and smell/ how long it had been there． If it was flesh /and smelled strongly， then /the ship was close to land． Birds Sea birds could be used to show the way/ to land /when it was nowhere to be seen． In the evening /nesting birds return to land /and their nests． So /seamen could follow the birds /to land /even if they were offshore/ and in the open sea． Using the weather Fog Fog gathers at sea /as well as over streams /or rivers． Seamen used it /to help identify the position of a stream /or river /when they were close to land． Winds Wise seamen used the winds/ to direct their sailing． They could accelerate the speed，but they could also be dangerous． So /the Vikings would observe the winds /before /and during their outward /or return journeys． Using the sea Certain tides/ and currents could be used /by skillful sailors /to carry ships/ to their destination. These skills helped sailors/ explore the seas/ and discover new lands． They increased their ability/ to navigate new seas/ when they used instruments． page 2: Using navigational instruments to help Finding longitude There was no secure method of measuring longitude /until the 17th century/ when the British solved this theoretical problem． Nobody knew that /the earth moved westwards/ 15 degrees/ every hour, but sailors did know an approximate method of calculating longitude /using speed /and time． An early method of measuring speed/ involved throwing a knotted rope/ tied to a log /over the side of the ship． The rope was tied to a log /which was then/ thrown into the sea． As the ship advanced through the water /the knots were counted/ as they passed through a seaman’s hands ． The number of knots/ that were counted /during a fixed period of time/ gave the speed of the ship/ in nautical miles /per hour． Later, when seamen began to use the pass /in the 12th century /they could calculate longitude /using plicated mathematical tables． The pass has a special magic pointer /which always indicates the North Pole， so/ it is used to help find the direction /that the ship needs to go． In this way /the ship could set a straight course /even in the middle of the ocean． Finding latitude The Bearing Circle It was the first instrument /to measure the sun39。s position． A seaman would measure the sun’s shad ow/ and pare it /with the height of the sun/ at midday． Then/ he could tell if he was sailing on his correct/ rather than a random course. The Astrolabe The astrolabe, quadrant/ and sextant are all connected. They are developments /of one another． The earliest， the astrolabe， was a special allinone tool /for telling the position of the ship/ in relation to the sun/ and various stars /which covered the whole sky． This gave the seamen /the local time /and allowed them /to find their latitude/ at sea． However, it was awkward/ to use /as one of the points of reference/ was the moving ship itself． The Quadrant This was a more precise/ and simplified version of the astrolabe． It measured how high stars were above the horizon /using a quarter circle/ rather than the full circle of the astrolabe． It was easier to handle /because it was more portable． Its shorting was that it still used the moving ship /as one of the fixed points of reference． As the ship rose /and plunged /in the waves， it was extremely difficult /to be accurate /with any reading． The sextant The sextant was the updated version /of the astrolabe /and quadrant/ which reduced the tendency/ to make mistakes． It proved to be the most accurate /and reliable of these early navigational instruments． It works by measuring the angle /between two fixed objects /outside the ship/ using two mirrors． This made the calculations more precise /and easier to do． III. Copying useful expressions and making sentences You are going to copy in your notebook all the useful expressions and make sentences of your own with them. sail the oceans, explore the oceans, plot a ship39。 已達到的： acplished facts 既成事實； 學識淵博的 , 技術高超的 , 有成就的： an acplished cook 廚藝精湛的廚師， Judy is acplished in English teaching. 茱蒂擅長英語教學； 有教養(yǎng)的 , 優(yōu) 雅的： an acpli