Emma. Surely then, papa, those stages which load their tops with a dozen or more people, cannot be safe for the passengers.

Father. They are very unsafe, but they would be more so, were not the roads about the metropolis remarkably even and good: and, in general, it is only within twenty or thirty miles of London, or other great towns, that the tops of carriages are loaded to excess.

Charles. I understand then, that the nearer the centre of gravity is to the base of a body, the firmer it will stand.

Father. Certainly; and hence you learn the reason why conical bodies stand so sure on their bases, for the tops being small in comparison of the lover parts, the centre of gravity is thrown very low and if the cone be upright or perpendicular, the line of direction falls in the middle of the base, which is another fundamental property of steadiness in bodies. For the broader the base, and the nearer the line of direction is to the middle of it, the more firmly does a body stand: but if the line of direction fall near the edge, the body is easily overthrown.

Charles. Is that the reason why a ball is so easily rolled along a horizontal plane?

Father. It is; for in all spherical bodies, the base is but a point, consequently almost the smallest force is sufficient to remove the line of direction out of it. Hence it is evident, that heavy bodies situated on an inclined plane will,

VOL. I.-E

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 в 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 (Pate 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

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 в; 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 в, 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.

Emma. I have sometimes trembled to pass by the hop-wagons which we have met on the Kent road.

Father. And without any impeachment of your courage, for they are loaded to such an enormous height, that they totter every inch of the road. It would indeed be impossible for one of these to pass with tolerable security along a road much inclined; the centre of gravity being removed so high above the body of the carriage, a small declination on one side or other would throw the line of direction out of the base.

Emma. When brother James falls about, is it because he cannot keep the centre of gravity between his feet?

Father. That is the precise reason why any person, whether old or young, falls. And hence you learn that a man stands much firmer with his feet a little apart than if they were quite close, for by separating them he increases the base. Hence also the difficulty of sustaining a tall body, as a walking cane, upon a narrow foundation.

Emma. How do rope and wire dancers, whom I have seen at the Circus, manage to balance themselves?

Father. They generally hold a long pole, with weights at each end, across the rope on which they dance, keeping their eyes fixed on some object, parallel to the rope, by which

means they know when their centre of gravity declines to one side of the rope or the other, and thus by the help of the pole, they are enabled to keep the centre of gravity over the base, narrow as it is. It is not however rope-dancers only that pay attention to this principle, but the most common actions of the people in general are regulated by it.

Charles. In what respects?

Father. We bend forward, when we go up stairs, or rise from our chair, for when we are sitting, our centre of gravity is on the seat, and the line of direction falls behind our base; we therefore lean forwards to bring the line of direction towards our feet. For the same reason a man carrying a burden on his back leans forward and backward if he carries it on his breast. If the load be placed on one shoulder he leans to the other. If we slip or stumble with one foot, we naturally extend the opposite arm, making the same use of it as the ropedancer does of his pole.

This property of the centre of gravity always endeavouring to descend, will account for appearances, which are sometimes exhibited to excite the surprise of spectators.

Emma. What are those, papa?

Father. One is, that of a double cone, appearing to roll up two inclined planes, forming an angle with each other, for as it rolls it sinks