Book IV chapter 7

Why the variation from that lateral¹ cause is not greater than has hitherto been observed, having been rarely seen to reach two points of the mariners²' compass, except near the pole.

T he earth, by reason of lateral eminences³ of the stronger globe, diverts iron and loadstone by some degrees from the true pole, or true meridian⁴. As, for example, with us English at London it varies eleven degrees and ⅓: in some other places the variation is a little greater, but in no other region is the end of the iron ever moved aside very much more from the meridian. For as the iron is always directed by the true verticity of the earth, so the polar nature of the continent land (just as of the whole terrene globe) acts toward the poles: and even if that mass divert magnetick bodies from the meridian, yet the verticity of those lands (as also of the whole earth) controls and disposes them so that they do not turn toward the East by any greater arc. But it is not easy to determine by any general method how great the arc of variation is in all places, and how many degrees and minutes it subtends on the horizon, since it becomes greater or less from diverse causes. For both the strength of true verticity of the place and of the elevated regions, as well as their distances from the given place and from the poles of the world, must be considered and compared; which indeed cannot be done exactly: nevertheless by our method the variation becomes so known that no grave error will perturb⁵ the course at sea. If the positions of the lands were uniform and straight along meridians⁶, and not defective⁷ and rugged⁸, the variations near lands would be simple; such as appear in the following figure.

This is demonstrated by a long loadstone the poles of which are in the ends A B; let C D be the middle line and the æquinoctial, and let G H and E F (the lines) be for meridians on which versoria are disposed, the variations of which are greater at a greater distance from the æquator. But the inequalities of the maritime⁹ parts of the habitable earth, the enormous promontories¹⁰, the very wide gulfs, the mountainous and more elevated regions, render the variations more unequal, or sudden, or more obscure; and, moreover, less certain and more inconstant in the higher latitude¹¹.

1 lateral   

adj.侧面的，旁边的

参考例句：

• An airfoil that controls lateral motion.能够控制横向飞行的机翼。
• Mr.Dawson walked into the court from a lateral door.道森先生从一个侧面的门走进法庭。

2 mariners   

海员，水手（mariner的复数形式）

参考例句：

• Mariners were also able to fix their latitude by using an instrument called astrolabe. 海员们还可使用星盘这种仪器确定纬度。
• The ancient mariners traversed the sea. 古代的海员漂洋过海。

3 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。

4 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。

5 perturb   

v.使不安，烦扰，扰乱，使紊乱

参考例句：

• Stellar passings can perturb the orbits of comets.行星的运行会使彗星的轨道发生扰动。
• They perturb good social order with their lie and propaganda.他们以谎言和宣传扰乱良好的社会秩序。

6 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

7 defective   

adj.有毛病的，有问题的，有瑕疵的

参考例句：

• The firm had received bad publicity over a defective product. 该公司因为一件次品而受到媒体攻击。
• If the goods prove defective, the customer has the right to compensation. 如果货品证明有缺陷， 顾客有权索赔。

8 rugged   

adj.高低不平的，粗糙的，粗壮的，强健的

参考例句：

• Football players must be rugged.足球运动员必须健壮。
• The Rocky Mountains have rugged mountains and roads.落基山脉有崇山峻岭和崎岖不平的道路。

9 maritime   

adj.海的，海事的，航海的，近海的，沿海的

参考例句：

• Many maritime people are fishermen.许多居于海滨的人是渔夫。
• The temperature change in winter is less in maritime areas.冬季沿海的温差较小。

10 promontories   

n.岬，隆起，海角( promontory的名词复数 )

参考例句：

11 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。