Sinan，the ancient Chinese compass
It was invented by Zhang Heng in the Han Dynasty.It is made up of a bronze foundation (青铜底盘) and magnetic spoon(磁勺) which is made of natural magnet，positioned right in the center of the foundation.
The handle of the spoon points to the south，while the other side points to the north.When the Sinan is in the still，the handle of the spoon points to the south because of the influence of the terrestrial magnetism (地磁)．
Astrolabe
It has 17 stars listed as well as a map of the sky overhead (for 37 degrees) and the ecliptic (黄道)．The time of day or night is on the outside edge with noon at the top of the Astrolabe and midnight at the bottom.It comes with a carrying chain，a pouch，and a book on its use.The reverse side has a sight for measuring the elevations of the heavenly bodies.
Sextant
The sextant is a navigational instrument for measuring the angle between the horizon and some object in the sky.First a sailor looks at the horizon through an eyepiece.At the see light from the sun or a star reflected off a small mirror on top of the instrument，onto a second mirror and into the eyepiece.
The navigator can then see two images，the horizon and the sun side by side.He can then measure the angle between them on a scale at the bottom of the instrument.The scale goes from 0 to 120 degrees.An earlier instrument，called a quadrant，measured in angle in the same way but it could only measure angles from 0 to 90 degrees.
Nautical chart
Nautical charts are maps of the depth of the sea and the currents of the oceans.They provide modern sailors with routes through the seas，rather like roads through the countryside.The information began to be collected in the nineteenth century and has continued to be updated ever since.These charts were not available to Captain Bligh or Zheng He.
两个诱思问题
1．What is the theory of the Sinan invented by Zhang Heng?
2．Why were the Nautical charts not able to be used by Captain Bligh or Zheng He?
【答案】
1．The influence of the terrestrial magnetism.
2．Because the information began to be collected in the nineteenth century.
Section Ⅰ Reading (Warming Up, Prereading，Reading & Comprehending)
