THE EARTH NOT QUITE ROUND.

“Father,” exclaimed James one day, “the[Pg 16] captain has been giving me a riddle¹ which I cannot guess. He asks where a pound of shot is not a pound. Is he joking with me?

Not at all, lad. A bag of shot weighing a pound in London, or even in Melbourne, will not weigh a pound on the equator².

What queer³ things you do tell me! How can that be?

You know that the world attracts you.

Yes; I know when I jump up I am soon brought down, and sometimes unpleasantly so, too.

As the world is round, and everywhere the world pulls things down to its surface, it is as if it were drawing all to the centre. This is called the Attraction of Gravitation.

And I suppose that like as fire is hotter the nearer you get to it, so does the attraction get greater the nearer you get to the centre of the earth.

True; but can you guess how this gravitation can be weight?

I know when I pull a thing, it is as if I pushed it, or put a weight on it. So the pulling of the earth will make the thing attracted feel as if a weight were on it.

And can you not see why a pound should somewhere not be a pound?

No; unless I made a big hole towards the centre, and then the pound weight would be heavier. But you told me the world was round, and it must be as far from the centre at London and Melbourne, as at the equator.

[Pg 17]But if from trial it be found that the pound really is less at the Line, what would it prove?

It proves what I can’t see, that the earth at the equator is farther off the centre than what it is at London or Melbourne, and that would make the earth not a true Globe.

Neither is it my son: though the difference is but little. It is about a dozen miles thicker down to the centre at the Line than at the poles.

Then, as London and Melbourne are each nearly half-way between the line and the pole they will be nearer by six miles to the centre than the equator is.

You have solved the problem, and understand that the world is not a sphere, but what is called an Oblate Spheroid, being flattened⁴ at the poles.

As I cannot easily find out by weighing that a pound is not a pound: can you tell me any other way of proving that the earth is not quite round, dear father?

Yes, I can. Do you see the reason why the pendulum⁵ returns down when it is moved up?

The gravitation, to be sure.

If a thing weighs heavier in one place than another through a difference of gravitation will there be a difference in the two places with the pendulum?

I should fancy that where the earth pulled the harder the pendulum would come down the quicker.

If it came down quicker it would rise quicker; that is, the beat would be quicker.[Pg 18] What effect would this have upon the clock?

The faster it beat, the faster the wheels would be moved, and the faster the hands would turn on the face.

Just so: the clock would be faster in time when the pendulum beat quicker, and slower in time when the pendulum beat slower.

But how could I find this out, father?

This way. It is ascertained⁶ that in London a pendulum must be 391?7 inches from the top to the middle of the swinging-bob to beat a true second at a time.

Then I am sure the London pendulum will not be pulled down so hard at the equator, and so it will be longer coming and going.

What must I do, then, to make my London pendulum beat seconds on the equator?

Why, shorten⁷ it a little bit.

Yes; or, what is the same thing, move up the weight at the end of the pendulum a little. I must shorten it one-tenth of an inch, or else it will lose sixteen hours in the year.

Then all the clocks carried to Melbourne from London will be wrong.

Will they go too fast or too slowly in Victoria?

They will be a little too slow, as Melbourne is nearer the equator than London is.

If a Melbourne clock-maker want his second pendulum to beat seconds in London what directions must he give?

He must tell the folks in London to lengthen⁸ the pendulum a bit.

You will see that by the world being[Pg 19] flattened at the poles the degrees of latitude⁹ will differ. One close to the equator measures 683?4 miles, in Victoria about 69 miles, in England 691?8, while in Lapland it would be 691?4.

What would be the mean length of a degree?

About 691?20 miles.

Let me multiply that by 360, to find the mean circumference¹⁰ of the world; I have it—24,858 miles.”

1 riddle   

n.谜，谜语，粗筛；vt.解谜，给…出谜，筛，检查，鉴定，非难，充满于；vi.出谜

参考例句：

• The riddle couldn't be solved by the child.这个谜语孩子猜不出来。
• Her disappearance is a complete riddle.她的失踪完全是一个谜。

2 equator   

n.赤道，（平分球形物体的面的）圆

参考例句：

• Singapore is near the equator.新加坡位于赤道附近。
• The United States is north of the equator.美国位于赤道以北。

3 queer   

adj.奇怪的，异常的，不舒服的，眩晕的

参考例句：

• I heard some queer footsteps.我听到某种可疑的脚步声。
• She has been queer lately.她最近身体不舒服。

4 flattened   

[医]（水）平扁的，弄平的

参考例句：

• She flattened her nose and lips against the window. 她把鼻子和嘴唇紧贴着窗户。
• I flattened myself against the wall to let them pass. 我身体紧靠着墙让他们通过。

5 pendulum   

n.摆，钟摆

参考例句：

• The pendulum swung slowly to and fro.钟摆在慢慢地来回摆动。
• He accidentally found that the desk clock did not swing its pendulum.他无意中发现座钟不摇摆了。

6 ascertained   

v.弄清，确定，查明( ascertain的过去式和过去分词 )

参考例句：

• The previously unidentified objects have now been definitely ascertained as being satellites. 原来所说的不明飞行物现在已证实是卫星。 来自《简明英汉词典》
• I ascertained that she was dead. 我断定她已经死了。 来自《简明英汉词典》

7 shorten   

vt.弄短，缩小，减少

参考例句：

• Minor accidents can shorten the life of a car.小事故会缩短汽车寿命。
• My dress is too long I must shorten it.我的连衣裙太长了，我得把它截短些。

8 lengthen   

vt.使伸长，延长

参考例句：

• He asked the tailor to lengthen his coat.他请裁缝把他的外衣放长些。
• The teacher told her to lengthen her paper out.老师让她把论文加长。

9 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。

10 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。