Here they changed horses, and their father desired the postilion to go slowly when he drove out of the town, and Harry and Lucy's heads were first out of one window, and then out of the other, eager for the first sight of the hanging tower.

"There it is!" cried Harry. "Of red brick!" said Lucy. "I see it. I had fancied that it was of grey old stone. I expected to see a fine venerable ivy-mantled tower."

"I cannot help it," said Harry; "but, my dear, take your head out of my way, for I cannot see what I want to see."

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There, you may have it all to yourself, my dear Harry," said Lucy. "I think it is very ugly."

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Nobody ever said it was pretty, did they?" said Harry; "but it is a curious thing."

"Not nearly so curious as I expected," said Lucy. "Not nearly so dangerous-looking as I fancied it must be. I had hoped that it would quite take away my breath to look at it, and that we could not drive by without fearing that it would fall upon our heads.”

Harry had no such fears or hopes, because he recollected distinctly having read an account of it. He knew that at Pisa there is a tower which overhangs 14 feet; much more than this of Bridgenorth. His father asked him if he could tell why these sloping towers continue to stand.

"What prevents them from falling, Harry, do you know?"

Harry said he believed he knew, because he had read in "Scientific Dialogues" an account of them, and an explanation of the principle on which they stand.

"I know you did," said his father, "and you understood it at the time you read it: but let me hear if you understand it now."

"Father, it is very difficult to express it in words, as Lucy says; but, if I had my little bricks, I could easily show the thing to you."

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Yes," said Lucy. "Very often we have built towers, which leaned over, and yet stood; and we used to try how much we could venture to make them overhang without their falling: I recollect that perfectly, though I do not know the reason why they did not fall.”

"If I had the little bricks, I could show and explain it," said Harry.

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But as you have them not, and as you cannot always carry a hod of little bricks about with you, Harry," said his father, "try what you can do to explain yourself by words; those you may always have at command."

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May-I wish I had," said Harry. "Begin, Harry, by thinking of what you wish to express, till you are quite clear that what you mean, and depend upon it you

you know

will then

"People often finding words,

easily find words," said his father. imagine that their difficulty is in when the real difficulty is in having clear ideas of things."

"Then you must, if you please, father, give me time," said Harry.

"As much time as you please," said his father; " and, that I may not hurry you, I will go on reading this book."

"Thank you, father," said Lucy for Harry.

Harry looked back at the hanging tower which was yet in view, and, after he had thought till he was clear of what he meant to say, without considering about the words, which came when he began to speak, he went on fluently, after the first word, “father," had made his father look up and lay aside his book.

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Suppose that a plumb-line was let down from the centre of gravity of the whole mass of that leaning tower, the bob of the plumb-line would fall within the base, or foundations: the plumbline could not hang outside of the base, or else the tower must tumble down. As long as the centre of gravity is supported, any part may lean over, or may hang out of the perpendicular line, and yet, provided the materials stick together, the tower will not fall."

"I think I understand that," said Lucy, "but I am not quite clear."

"If you do not understand what is meant by the centre of gravity," said her father, "you cannot be quite clear, indeed you cannot comprehend it at all."

Lucy said she remembered seeing Harry, when he was a little boy, standing upon her father's knees, and leaning his body so much to one side, that she was afraid he must have fallen. And, papa, you then explained to me how far he might have leaned over without any risk of tumbling down. You also told me something about the centre of gravity, but that I do not recollect, exactly."

"Remember how often," said Harry, "my father has shown us that the motions of our tumbler toy depended on the situation of the centre of gravity. By-the-bye, there is a way of finding out where the centre of gravity is in a body or figure of any shape."

"Is there?" said Lucy. "I do not remember papa's showing us that. How did you learn it? And, if it is not very difficult, can you teach it to me? Who taught it to you?"

"A book," said Harry; "my own good 'Scientific Dialogues.' And if I had but a bit of card, and a bit of thread, and a pin, and a pencil, and if we were not in the carriage, I think I could show it to you now."

But, as all these ifs stood in the way, the matter was put off for the present, and, like many good

things that are put off, was forgotten. Nor can we much wonder that this was driven from the recollection even of the philosophic Harry, by the bustle he saw in the next town through which they passed. It happened that there was a great fair in this town, and it was filled with such crowds of people, and so many stalls and booths, covered with canvass, lined the streets, that it at first seemed impossible that way for their carriage and horses could be made through them, without trampling on some of the people, or overturning some of the tents. The postilion stopped, and called civilly to the people to make way: cloaks and great coats yielded on each side, and those in front got from under the horses' noses, as they advanced step by step. Lucy had let down the glasses, and was looking out with great interest, not unmixed with apprehension, and listening to the—

"Universal hubbub wild

Of stunning sounds and voices all confused."

And as, little by little, the carriage made its way through the dense multitude, she saw many hairbreadth escapes, which made her shrink, and cry "Oh!" and "Ah!" many a time for those who were all unheedful of their danger, in the ardour of bargaining, the pleasure of gossiping, the vehemence of scolding, the stupidity of staring, or the anxiety of maternal affection. Here was seen