344 Pleasant Ways in Science. 



In these experiments we suspend a body, free to move 

 about the point of suspension, so that it may be at rest in any 

 position ; and we see that to do so, we must suspend it by a 

 support passing through its centre of gravity. Let us now 

 suppose we wish to communicate motion to any body, so that 

 all its parts shall be impelled in the same direction, and with 

 uniform velocities ; how are we to proceed ? Take a billiard 

 ball for an example. If one side of the ball is struck, that 

 side is impelled to move faster than the other ; but if the ball 

 is hit exactly in the centre, its tendency is to move straight 

 forward in the direction of the impelling force. Thus we see 

 that, if an object is struck, so that the line of the force acting 

 upon it passes exactly through its centre of gravity, the whole 

 body is impelled to move straight forward with equal velocities 

 affecting all its parts. If struck to the right of its centre of 

 gravity, one side is impelled to move quicker than the other, 

 and consequently the body rotates round its centre of gravity 

 as well as moves forwards, if it is free to do so. The centre of 

 gravity of a body has therefore two noticeable properties — a 

 support passing through it will supend the body, so that its 

 balance is not disturbed by change of position, and a force 

 passing through it impels the whole body to move equably 

 forward in front of the impelling force. 



A body is in a state of equilibrium when the action of 

 gravitation does not tend to alter its position ; but there are 

 three distinct kinds of equilibrium — stable, unstable, and 

 neutral. Stable equilibrium indicates a decided preference for 

 a particular position of equilibrium. This is the case when a 

 cone is allowed to stand on its base. If you lift the base up a 

 little on one side, it falls back to its previous position ; and in 

 order to make it fall over, you must lift one side of the base so 

 much, that a perpendicular from the cone's centre of gravity 

 shall fall beyond the base, and then it will fall over on its side. 

 Any body in stable equilibrium has its centre of gravity so far 

 from the edge of its base, that if thrown slightly, or even 

 considerably, out of position, it tends to fall back to where it 

 was before. A centre of gravity is in its normal position when 

 it has fallen as low — that is, as near the centre of the earth — 

 as it can. If elevated above the lowest point it can reach, and 

 allowed freedom for motion, it will get back to the lowest 

 point by the shortest route. In the annexed diagram, let the 

 three triangles represent three cones. Their centres of gravity 

 will be exactly over the centre of the base, and one quarter of 

 the distance from the vertex to the base above it. In A a 

 perpendicular from the centre of gravity is considerably distant 

 from either edge of the base. In B we see it displaced, but 

 its shortest way of getting to the lowest point is to fall to the 



