Book IV chapter 4

The arc of variation is not changed equally in proportion to the distance of places.

I n the open sea, when a vessel¹ is borne by a favourable² wind along the same parallel, if the variation be changed by one degree in the course of one hundred miles, the next hundred miles do not therefore lessen³ it by another degree; for the magnetick [needle] varies erratically⁴ as respects position, form, and vigour⁵ of the land, and also because of the distance. As, for example, when a course from the Scilly Isles⁶ to Newfoundland has proceeded so far that the compass is directed to the true pole, then, as the vessel proceeds, in the first part of the course the variation increases toward the north-west222, but rather indistinctly and with small difference: thence, after an equal distance, the arc is increased in a greater proportion until the vessel is not far from the continent: for then it varies most of all. But before it touches actual land or enters port, then at a certain distance the arc is again slightly diminished. But if the vessel in its course should decline greatly from that parallel either toward the south or the north, the magnetick [needle] will vary more or less, according to the position of the land and the latitude⁷ of the region. For (cæteris paribus) the greater the latitude the greater the variation.

1 vessel   

n.船舶；容器，器皿；管，导管，血管

参考例句：

• The vessel is fully loaded with cargo for Shanghai.这艘船满载货物驶往上海。
• You should put the water into a vessel.你应该把水装入容器中。

2 favourable   

adj.赞成的，称赞的，有利的，良好的，顺利的

参考例句：

• The company will lend you money on very favourable terms.这家公司将以非常优惠的条件借钱给你。
• We found that most people are favourable to the idea.我们发现大多数人同意这个意见。

3 lessen   

vt.减少，减轻；缩小

参考例句：

• Regular exercise can help to lessen the pain.经常运动有助于减轻痛感。
• They've made great effort to lessen the noise of planes.他们尽力减小飞机的噪音。

4 erratically   

adv.不规律地，不定地

参考例句：

• Police stopped him for driving erratically. 警察因其驾驶不循规则而把他拦下了。 来自辞典例句
• Magnetitite-bearing plugs are found erratically from the base of the Critical Zone. 含磁铁岩的岩栓不规则地分布于关键带的基底以上。 来自辞典例句

5 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。

6 isles   

岛( isle的名词复数 )

参考例句：

• the geology of the British Isles 不列颠群岛的地质
• The boat left for the isles. 小船驶向那些小岛。

7 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。