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Fig.9

Fig.15.

A

ine a в be hung

ough the centre

ot in the lowest ne as the plumb ang the body up the plumb line gh the centre of as before; and ity is somewhere of DE, it must be es cross.

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ople who have to light goods, as to find the centre

ality of them never d it seems surpriseless make their up

to keep the line of dle of the base.

while the line of direction falls within the base, slide down upon the plane: but they will roll when that line falls without the base. The body A (Plate 1. Fig. 8.) will slide down the plane DE, but the bodies B and c will roll down it.

Emma. I have seen buildings lean very much out of a straight line, why do they not fall?

Father. It does not follow, because a building leans, that the centre of gravity does not fall within the base. There is a high tower at Pisa, a town in Italy, which leans fifteen feet out of the perpendicular; strangers tremble to pass by it, still it is found by experiment that the line of direction falls within the base, and therefore it will stand while its materials hold together.

A wall at Bridgenorth, in Shropshire, which I have seen, stands in a similar situation, for so long as a line cb (Plate 11. Fig. 9.) let fall from the centre of gravity c of the building AB, passes within the base CB, it will remain firm, unless the materials with which it is built go to decay.

Charles. It must be of great use in many cases to know the method of finding the centre of gravity in different kinds of bodies.

Father. There are many easy rules for this with respect to all manageable bodies; I will mention one, which depends on the property which the centre of gravity has, of always endeavouring to descend to the lowest point.

If a body A (Plate 11. Fig. 10.) be freely sus

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pended on a pin a, and a plumb line a в be hung by the same pin, it will pass through the centre of gravity, for that centre is not in the lowest point, till it fall in the same line as the plumb line. Mark the line a B; then hang the body up by any other point, as D, with the plumb line DE, which will also pass through the centre of gravity for the same reason as before; and therefore as the centre of gravity is somewhere in a B, and also in some point of DE, it must be in the point c where those lines cross.

CONVERSATION X.

Of the Centre of Gravity.

Charles. How do those people who have to load carts and wagons with light goods, as hay, wool, &c. know where to find the centre of gravity?

Father. Perhaps the generality of them never heard of such a principle; and it seems surprising that they should nevertheless make up their loads with such accuracy as to keep the line of direction in or near the middle of the base.

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