Part 7

But how could I find this out, father?

This way. It is ascertained that in London a pendulum must be 39-1/7 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, shorten 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 lengthen the pendulum a bit.

You will see that by the world being flattened at the poles the degrees of latitude will differ. One close to the equator measures 68-3/4 miles, in Victoria about 69 miles, in England 69-1/8, while in Lapland it would be 69-1/4.

What would be the mean length of a degree?

About 69-1/20 miles.

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

ANNUAL MOTION.

The ship was much delayed in the Region of the Calms, as it is called; that is, near the equator.

After a while the astronomical dialogue was resumed.

“Well, James, look now at your twinkling friends, and tell where you are.

I suppose by the North Polar star resting upon the waters to the north, that we must be now nearly on the top of the hill, by the equator.

We are so, my lad. But what time did the sun rise this morning?

At six, and it set at exactly six this evening.

Do you know what day of the month it is?

The twenty-third of September.

Have you ever observed the sun to rise and set at six o’clock before, on the voyage?

No, father.

Well, then, you will recollect hereafter that you were upon the equator on the twenty-third of September, when there was equal day and night. But did you not know of equal day and night at home?

Yes; about half-way between winter and summer.