the air as it was whirled round while soft, as any other substance is made circular by the pressure of the tool when turning round in a lathe. And he thought that the air withinside of the glass prevented its being driven in, and squeezed together by the motion.

His father told him that he was partly right in his supposition; but there was one reason, one cause, to which he had not adverted, and which he had not yet perceived. He would say no more, because, perhaps, by observing further, he would discover it for himself, in attending to another operation the blowing of window glass, or crown glass, as it is called. First, a great pear-shaped bubble of glass, about a foot in diameter, was blown at the end of an iron tube, to which, being soft and hot, it adhered. Then, by rolling the pearshaped bulb upon a smooth marble table, and blowing into it, and by repeating these operations alternately several times, and by whirling it rapidly round near a hot fire, the bulb was brought from its pear shape

into that of a globe. The globe was then cut open opposite to the iron tube, and after being whirled again with great swiftness, the parts spread out still more, till, by continuing this operation, it became a large circular plate of flat glass.

Harry now perceived what he had omitted in the case of the drinking glass, the centrifugal force, or that force which arises from the tendency the parts of bodies have to fly from their centre, when turned round rapidly.

As he left the glass house, he continued his explanation.

"I suppose, father, that the parts of the soft glass, as they are whirled round, endeavoured to fly from the centre, and by so doing the globe became a larger globe, and the circular plates became larger circles, and all the parts flying off equally from the centre, the rim of the drinking glass became quite circular."

"It is," said Lucy, "not exactly, but something like a mop. Yes, Harry, a

mop. When the maid twirls it round

fast, the threads of the mop all fly out as far as they can go from the centre; and if it was a wet mop, out fly circles beyond circles of drops."

"Well," said Harry, " you have made out your likeness to a mop better than I thought you could."

"I remember," continued Lucy, "the first day I ever heard of centrifugal force, or had any idea of what it meant; it was from you, Harry; when I was making a pancake, papa."

"A pancake, my dear! I do not remember your ever making a pancake."

Perhaps it was a cheese," said Lucy. "Some people, I believe, call it a cheese. Not a cheese or a pancake to eat, papa; but I will show you as soon as we are in mamma's room."

Lucy kept her word, and whirling herself round the moment she was in her mother's room, the skirts of her petticoats flew out, and, as she popped downwards, while they swelled out, she ex claimed

"There is a pancake, papa, or a cheese, whichever you please; and it is made, Harry, by centrifugal force, is not it?"

"I have been very much amused," continued Lucy, "seeing the glass-blowing. Were not you, Harry?”

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Very much, indeed, and it has left a great deal to think of, and to inquire more about," said Harry.

"What more?" said Lucy.

"A great deal," repeated Harry. "For one thing, annealing, I do not understand that."

"I recollect," said Lucy, "that when the man had twirled the wine glass round, and finished it, a boy came with a long pair of tongs, and seizing the glass ran away with it, as our man said, to the annealing furnace to be annealed. And when I asked what that was, and what was to be done more to it, the man showed me a pan in an oven, and I saw our wine glass, with many others, put into it to be heated again, and then to be left to cool slowly. The man told me they ought to take several

days to cool. This was done to make the glass less brittle, he said; and this is annealing. What more, Harry, would you know about it?"

"A great deal more, if I could," said Harry. In the first place, I do not in the least know why annealing makes glass less brittle."

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Why? Oh! that is another affair. Why? Nor I," said Lucy.

"And I heard papa and the master-man in the glass house talking of a curious fact. He said, that when a glass vessel, of a particular shape, is allowed to cool immediately after being made, it will often sustain the shock of a pistol bullet, or any other blunt body falling into it from a considerable height; but a small splinter of flint, dropped gently into it, makes it fly to pieces with great violence.""

"Indeed," said Lucy,

curious."

"that is very

"So papa said; and they went off to talk about Prince Rupert's drops. Oh, my dear, there are a great many, many