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not always, attached a train, having a like appearance. The former is called the head, and the latter the tail, of the comet. When it comes nearest to the sun, it becomes visible, and afterwards departs into remote regions and disappears. Q. Of what shape are comets?

A. None of them have been ascertained to have any definite shape. Q. Are they solid bodies?

A. It is certain that many of them possess no solid matter, but are masses consisting of some nearly transparent substances; others are so surrounded by this apparently vapourous matter, that it is impossible, by any means of observation which we possess, to discover whether this vapour enshrouds within it any solid mass.

Q. What are the tides?

A. They are the alternate rising and falling of the waters of the ocean, and of the bays, friths, and rivers connected with the ocean.

Q. At what interval does that alternate rising and falling take place?

A. Twice in about 24 hours 49 minutes; that is to say, high tide takes place twice during that period, and low tide also twice.

Q. Describe the phenomenon of the rising and falling, or flux and reflux, of the water of the ocean.

A. If, for instance, in a harbour, the water is now at its greatest height, it will presently begin to fall; and this falling continues for about 6 hours and 12 minutes, at the end of which the water will be at its lowest.

It then be

gins to rise, and the rising likewise continues for about 6 hours and 12 minutes, when it attains its greatest height. It then begins to fall for about 6 hours and 12 minutes, and then rises for the same time; so that, in the space of about 24 hours and 49 minutes, the water rises and falls twice, and arrives alternately at its greatest and least depth.

Q. What is a flood tide?

A. When the water is rising, it is called a flood tide.

Q. What is an ebb tide?

A. When the water is falling, it is called an ebb tide.

Q. What causes the tides?

A. They are caused by the attraction of the sun and moon; but principally by the attraction of the moon.

Q. How does it appear that the tides are caused principally by the moon's action?

A. If high water happen to-day at noon, it will be high water tomorrow about 49 minutes after noon; so that, from one day to the next the tides are later by about 49 minutes. The same thing takes place in the motion of the moon, which rises each day about 49 minutes later than on the preceding day.

Q. How is it that the moon, which is so much smaller than the sun, produces a more powerful effect on the tides than the latter?

A. Because the moon is so much nearer to the earth than the sun.

Q. How much greater influence does the moon produce on the tides than the sun?

A. It is calculated that the influence of the moon on the tides is three times as great as the influence of the sun. Q. What are spring tides?

A. They are the highest tides, and take place at the seasons of new and full moon, when the influence of the sun and moon on the tides is united.

Q. What are neap tides?

A. They are the lowest tides, and take place when the moon is in her quadratures or quarters. At that period the influence of each of the luminaries on the tides is directly opposed to that of the other.

Q. Do the tides rise to the same height in different places?

A. No; in some places they rise only a few feet, while in others they rise as much as 40 feet and upwards.

Q. What causes this difference?

A. The obstacles offered by the land to the ebb and flow of the waters.

ELECTRICITY,

Q. What is electricity?

A. If a tube of glass, or a piece of sealing-wax, perfectly dry, be rubbed against dry woollen cloth or silk, and afterwards held over small and light bodies, such as pieces of paper, thread, or cork, strewed upon a table, these will be seen to fly towards the glass or sealing-wax, and stick to it for a short time. The cause of this attraction is called Electricity.

Q. What is the word electricity derived from?

A. It is derived from elektron, a Greek word signifying amber; because amber was the first substance which, by being rubbed, was observed to attract light bodies.

Q. What is the electric spark?

A. If a ball made of metal, or the knuckle, be presented to another body, in which a sufficient amount of electri

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