Book IV chapter 2

That the variation is caused by the inæquality of the projecting parts of the earth.

D emonstration of this may manifestly be made Variation on imperfect terrella. by means of a
terrella in the following way: let there be a round loadstone somewhat imperfect in some part, and impaired¹ by decay (such an one we had with a certain part corroded² to resemble the Atlantick or great Ocean): place upon it some fine iron wire of the length of two barleycorns, as in the following figure. A B, a Terrella in certain parts somewhat imperfect and of unæqual virtue³ on the circumference⁴. The versoria E, F, do not vary, but look directly to the pole A; for they are placed in the middle of the firm and sound part of the terrella and somewhat distant from the imperfect part: that part of the surface which is distinguished⁵ by dots and transverse lines is the weaker. The versorium O also does not vary (because it is placed in the middle of the imperfect part), but is directed toward the pole, just as near the western Azores on the earth. The versoria H and L do vary, for they incline toward the sounder parts very near them. As this is manifest in a terrella whose surface is sensibly rather imperfect, so also is it in others whole and perfect, when often one part of the stone has stronger external parts, which nevertheless do not disclose themselves manifestly to the senses. In such a terrella the demonstration⁶ of the variation and the discovery of the stronger parts is on this wise. Let A be the pole, B the place of the variation, C the stronger regions; then the horizontal versorium at B varies from the pole A toward C: so that both the variation is shown and the stronger places of the loadstone recognized. The stronger surface is also found by a fine iron wire of the length of two barleycorns: for since at the pole of the terrella it rears up perpendicularly⁷, but in other places inclines toward the æquator, if in one and the same parallel circle it should be more erect⁸ in one place than in another; where the wire is raised more upright, there the part and surface of the terrella is stronger. Also when the iron wire placed over the pole inclines more to one part than to another. Let the experiment be made by means of a fine iron wire of three digits⁹ length placed over the pole A, so that its middle lies over the pole. Then one end is turned away from B toward C, and is not willing to lie quietly toward B; but on a terrella which is perfect²²⁰ all round and even it rests on the pole directed toward any point of the æquator you please. Otherwise, let there be two meridians¹¹ meeting in the poles A B, let iron wires be reared just at the ends D and C of the equal arcs D A and C A; then the wire at D (the stronger region) will be more raised up than that at C, the weaker. And thus the sounder and stronger part of the loadstone is recognized, which otherwise would not be perceived by the touch. In a terrella which is perfect, and even, and similar in all its parts, there is, at equal distances from the pole, no variation²²¹. Variation is shown by means of a terrella, a considerable part of which, forming a surface a little higher than the rest, does, although it be not decayed and broken, allure¹² the versorium from the true direction (the whole terrella co-operating).

uneven¹³ in surface." alt="A terrella uneven in surface." src="https://ebooks.adelaide.edu.au/g/gilbert/william/on-the-magnet/figures/illo142.jpg" />

A terrella uneven in surface.

 

It is shown by a small spike¹⁴ placed over a terrella or by a small versorium; for they are turned by the terrella toward the mass that stands out and toward the large eminences¹⁵. In the same way on the earth the verticity is perturbed¹⁷ by great continents, which are mostly elevated above the depths of the seas and make the versorium deviate¹⁸ sometimes from the right tracks (that is, from the true meridians). On a terrella it is thus demonstrated: the end of the versorium A is not directed straight to the pole P, if there be a large protuberance B on the terrella; so also the cusp C deviates¹⁹ from the pole because of the eminence¹⁶ F. In the middle between the two eminences the versorium G collimates to the true pole because, being at equal distances from the two eminences B and F, it turns aside to neither, but observes the true meridian¹⁰, especially when the protuberances are of equal vigour²⁰. But the versorium N on the other side varies from the pole M toward the eminences H, and is not held back, stopped, or restrained by the small eminence O on the terrella (as it were, some island of land in the ocean). L, however, being unimpeded, is directed to the pole M. The variation is demonstrated in another way on a terrella, just as on the earth. Let A be the pole of the earth, B the equator, C the parallel circle of latitude²¹ of 30 degrees, D a great eminence spread out toward the pole, E another eminence spread out from the pole toward the æquator. It is manifest that in the middle of D the versorium F does not vary; while G is very greatly deflected²²: but H very little, because it is further removed from D. Similarly also the versorium I placed directly toward E does not deviate from the pole: but L and M turn themselves away from the pole A toward the eminence E.

1 impaired   

adj.受损的；出毛病的；有（身体或智力）缺陷的v.损害，削弱( impair的过去式和过去分词 )

参考例句：

• Much reading has impaired his vision. 大量读书损害了他的视力。 来自《现代汉英综合大词典》
• His hearing is somewhat impaired. 他的听觉已受到一定程度的损害。 来自《现代汉英综合大词典》

2 corroded   

已被腐蚀的

参考例句：

• Rust has corroded the steel rails. 锈侵蚀了钢轨。
• Jealousy corroded his character. 嫉妒损伤了他的人格。

3 virtue   

n.德行，美德；贞操；优点；功效，效力

参考例句：

• He was considered to be a paragon of virtue.他被认为是品德尽善尽美的典范。
• You need to decorate your mind with virtue.你应该用德行美化心灵。

4 circumference   

n.圆周，周长，圆周线

参考例句：

• It's a mile round the circumference of the field.运动场周长一英里。
• The diameter and the circumference of a circle correlate.圆的直径与圆周有相互关系。

5 distinguished   

adj.卓越的，杰出的，著名的

参考例句：

• Elephants are distinguished from other animals by their long noses.大象以其长长的鼻子显示出与其他动物的不同。
• A banquet was given in honor of the distinguished guests.宴会是为了向贵宾们致敬而举行的。

6 demonstration   

n.表明，示范，论证，示威

参考例句：

• His new book is a demonstration of his patriotism.他写的新书是他的爱国精神的证明。
• He gave a demonstration of the new technique then and there.他当场表演了这种新的操作方法。

7 perpendicularly   

adv. 垂直地, 笔直地, 纵向地

参考例句：

• Fray's forehead was wrinkled both perpendicularly and crosswise. 弗雷的前额上纹路纵横。
• Automatic resquaring feature insures nozzle is perpendicularly to the part being cut. 自动垂直功能，可以确保刀头回到与工件完全垂直的位置去切割。

8 erect   

n./v.树立，建立，使竖立；adj.直立的，垂直的

参考例句：

• She held her head erect and her back straight.她昂着头，把背挺得笔直。
• Soldiers are trained to stand erect.士兵们训练站得笔直。

9 digits   

n.数字( digit的名词复数 )；手指，足趾

参考例句：

• The number 1000 contains four digits. 1000是四位数。 来自《简明英汉词典》
• The number 410 contains three digits. 数字 410 中包括三个数目字。 来自《现代英汉综合大词典》

10 meridian   

adj.子午线的；全盛期的

参考例句：

• All places on the same meridian have the same longitude.在同一子午线上的地方都有相同的经度。
• He is now at the meridian of his intellectual power.他现在正值智力全盛期。

11 meridians   

n.子午圈( meridian的名词复数 )；子午线；顶点；（权力，成就等的）全盛时期

参考例句：

• Meridians are great circles passing through both poles. 经线均为通过两极。 来自辞典例句
• The Cutaneous Regions are within the domains of the Twelve Regular Meridians. 十二皮部是十二经脉功能活动反映于体表的部位，也是络脉之气散布之所在。 来自互联网

12 allure   

n.诱惑力，魅力；vt.诱惑，引诱，吸引

参考例句：

• The window displays allure customers to buy goods.橱窗陈列品吸引顾客购买货物。
• The book has a certain allure for which it is hard to find a reason.这本书有一种难以解释的魅力。

13 uneven   

adj.不平坦的，不规则的，不均匀的

参考例句：

• The sidewalk is very uneven—be careful where you walk.这人行道凹凸不平—走路时请小心。
• The country was noted for its uneven distribution of land resources.这个国家以土地资源分布不均匀出名。

14 spike   

n.长钉，钉鞋；v.以大钉钉牢，使...失效

参考例句：

• The spike pierced the receipts and held them in order.那个钉子穿过那些收据并使之按顺序排列。
• They'll do anything to spike the guns of the opposition.他们会使出各种手段来挫败对手。

15 eminences   

卓越( eminence的名词复数 )； 著名； 高地； 山丘

参考例句：

• Both are truly grey eminences who have become accustomed to the exercise of influence. 两个人都是真正的幕后操纵者，他们已习惯于用其影响进行幕后操纵。

16 eminence   

n.卓越，显赫；高地，高处；名家

参考例句：

• He is a statesman of great eminence.他是个声名显赫的政治家。
• Many of the pilots were to achieve eminence in the aeronautical world.这些飞行员中很多人将会在航空界声名显赫。

17 perturbed   

adj.烦燥不安的v.使（某人）烦恼，不安( perturb的过去式和过去分词 )

参考例句：

• I am deeply perturbed by the alarming way the situation developing. 我对形势令人忧虑的发展深感不安。 来自《简明英汉词典》
• Mother was much perturbed by my illness. 母亲为我的病甚感烦恼不安。 来自《现代英汉综合大词典》

18 deviate   

v.(from)背离，偏离

参考例句：

• Don't deviate from major issues.不要偏离主要问题。
• I will never deviate from what I believe to be right.我绝不背离我自信正确的道路。

19 deviates   

v.偏离，越轨( deviate的第三人称单数 )

参考例句：

• The boy's behavior deviates from the usual pattern. 这个男孩子的举止与一般人不同。 来自《简明英汉词典》
• The limit occurs when the ordinate deviates appreciably from unity. 这个限度发生在纵坐标明显地从单位1偏离的时候。 来自辞典例句

20 vigour   

（=vigor）n.智力，体力，精力

参考例句：

• She is full of vigour and enthusiasm.她有热情，有朝气。
• At 40,he was in his prime and full of vigour.他40岁时正年富力强。

21 latitude   

n.纬度，行动或言论的自由(范围)，(pl.)地区

参考例句：

• The latitude of the island is 20 degrees south.该岛的纬度是南纬20度。
• The two cities are at approximately the same latitude.这两个城市差不多位于同一纬度上。

22 deflected   

偏离的

参考例句：

• The ball deflected off Reid's body into the goal. 球打在里德身上反弹进球门。
• Most of its particles are deflected. 此物质的料子大多是偏斜的。