21.Theory of Magnetism.
Experiment 16. -Heat a No. 20 knitting needle red hot and plunge it quickly into cold water. This tempers the needle so that it will break readily. Magnetize the needle as was done in Experiment 11. When it has become well magnetized， break it in the middle. Test each half with a suspended magnet， as was done in Experiment 13. Is each half a full magnet or only half a magnet？ Break these halves again and test. What effect does breaking a magnet have upon the magnet？
In Experiment 16 it was found that if a magnet is broken in two， each half is a perfect magnet. If these halves are broken， each piece is a perfect magnet， and so on as long as the division is kept up. It is also found that if a magnet is heated or suddenly jarred or pounded it loses its magnetism. If a magnet is filed into filings and these filings are put into a glass tube the tube will have no magnetic properties but will act to a magnet like an ordinary iron bar.
If now the tube is held vertically and tapped several times on a strong magnet， the tube will be found to have acquired the properties of a magnet. The tapping joggled the particles so that they could arrange themselves under the influence of the magnetic pole and whenthey became so arranged a magnet was the result. If the filings are now poured out of the tube and then put back again， there will be no magnetization.
It was the arrangement of the tiny magnetized particles which must have caused the contents of the tube to become magnetic. It would therefore seem probable that magnetism must be a property of the exceedingly small particles or molecules of which the iron or steel as well as all other substances are supposed to be composed.
It is supposed that when a bar of steel becomes magnetized the molecules arrange themselves in definite directions， as do the filings in the tube. The molecules of magnetic substances are supposed to be separate little magnets. In the unmagnetized bar (Fig. 1 7) their poles point in all directions dependent upon their mutual attraction， and thus they neutralize each other. When the bar becomes magnetized the molecules tend to arrange themselves so that like poles lie in the same direction (Fig. 18). When the magnet is heated or jarred the molecules are moved out of thisalignment and the magnetism is weakened.
Fig. 17.
Fig. 18.
Summary. -The shape of the earth is spherical with very slight flattening at the poles. Its diameter is almost 8000 miles， more than four times the distance from New York to Denver； and its circumference is nearly 25，000 miles. Though the irregularities of the earth"s surface are exceedingly small in comparison with its total area， they are great enough to have a vast effect upon the life of animals and plants.
The revolution of the earth upon its axis causes day and night， and gives us our measurement of time and our points of the compass. The movement of the earth around the sun， combined with the inclinationof the earth"s axis， gives us the seasons. The inclination of the earth"s axis， combined with its revolution， and the movement just mentioned， gives us our latitude zones and causes the variation in the length of our days and nights.
Distance east and west on the earth is measured by meridians of longitude； distance north and south by parallels of latitude. The parallels are about seventy miles apart， and this is also about the distance between the meridians at the equator. Two parallels at the poles are about the same distance apart as two at the equator. But as all meridians pass through the poles， there is no distance at all between them there， while at the equator they are about seventy miles apart.
To find the direction a ship sails we use a compass， a magnetized needle which points toward the north magnetic pole， located northeast of Alaska. Thus， the magnetism of the earth is of infinite value to ocean commerce.
QUESTIONS
What simple reasons are there for believing that the earth is round？ What effects have the irregularities of the earth"s surface had on life？ Draw diagrams illustrating what was discovered in Exp. 8.
Why do we have winter when the earth is nearest the sun？
If a man left Cairo， Egypt， on June 21 and traveled slowly to Cape Town， reaching there on Dec. 21， what changes of seasons would he experience？
How is the length of a day determined？ If it were noon Thursday， Sept. 30， with you， what would be the day and date at Yokohama？
Why is Standard Time particularly advantageous in the United States？ How much is the difference between local and standard time at your locality？
Suppose that a man at the north pole traveled a degree south and then a degree east. How far would he travel？ Suppose he were a degree north of the equator and traveled a degree south and then a degree east. How far would he travel？ In both cases he traveled a degree of latitude and a degree of longitude. Do the distances differ？ If so， why？
If it is 12 o"clock local time at your home， what time is it at Paris？ At Honolulu？
What practical advantages and applications of magnetism do you know？ Why is it necessary for a mariner to know the declination？
【中文阅读】
5.地球的形状--用各种方法精确测量了地球形状的人告诉我们，地球的 形状像一个扁球，好比一个皮球在两端有点泄气凹瘪的样子。一个普通的橘子 类似于这个形状，但是地球两极的凹瘪程度非常小，所以总体看来地球更像一 个圆球而不像一个橘子，它的两极直径只比其赤道直径短了27英里，因此除非 是在极其精确测量的场合，我们都认为地球总体直径就是8000英里，个别地方 短出来的那27英里可以忽略不计。
实验6：将一个软皮球装在旋转装置上让其高速旋转，可以发现运动幅度 最大的球的中心段鼓了起来，而运动幅度最小的转轴两端却瘪了下去，地球的 自转与此非常相似，并且在两极处有轻微凹瘪。
即使地球上有超过海拔5英里的高山，海底还有比这还深的海沟，但这些 参差的长度和地球的尺寸比起来太微不足道了，所以总体来看，地球的表面比 橘子表面还要规则平整一些。
现如今很多人已经做过环球航行了，但要说真正坚毅勇敢的，还是要算 在1522年首次证明沿着一个方向航行也可以回到起点的那一船勇者，这是第一 次证明了地球可能是个圆球。在很久以前，一些智者已经开始相信地球是个球 体，这至少可以追溯到亚里士多德时代，这位古希腊著名的哲学家就这么认 为，因为他注意到发生月食的时候，月亮的阴影边界总是圆弧形的。后来还有 人注意到，从茫茫大海上驶来的帆船，总是先出现桅杆然后才渐渐依次往下出 现整个船身；在帆船驶离我们视线的时候，整个过程正好相反。
实验7：将适量酒精兑入水中，得到一个让普通油液可以在其中悬浮的溶 液。把一根长玻璃管插入溶液中，滴入一大滴油液，让油滴在溶液表面下悬浮 成一个小圆球。这个现象说明液体在减轻外部作用力之后，整体会呈现为标准 的球形。