Pleasant Ways in Science. 



345 



right, so as to bring the cone back to the position shown in A. 

 In 0, the nearest way for the centre of gravity to reach its 

 lowest point is by the cone falling on its side towards the right ; 

 and it will be seen that a perpendicular from the centre of 

 gravity falls in the same direction to the right of the base and 

 outside it. A body like A is in a stable equilibrium, because a 

 moderate or considerable disturbance leaves it able to recover 

 its position. If C were balanced exactly upon the point D, it 

 would be in unstable equilibrium, because the slightest force 

 would make it fall on either side. 



. Neutral equilibrium is when a body cannot alter the position 

 of its centre of gravity by any motion it can take. It is thus 

 equally at rest, or in equilibrium, in all its possible positions. 

 A ball resting on a plane is iu this condition, because its 

 revolution leaves its centre of gravity at all times exactly as 

 high above the plane as it was before. 



To return to our scales : let the piece of card already spoken 

 of represent the beam. If the needle by which it is suspended 

 passes exactly through its centre of gravity, it is in neutral 

 equilibrium, and the two arms balance each other in whichever 

 way they are placed. But if the needle is moved so as to be 

 a little above the centre of gravity, a slight motion of the beam 

 elevating or depressing either arm is resisted by its weight, 

 because any such motion lifts the centre of gravity above its 

 lowest point, to which it will immediately fall back, and at 

 which it will settle after a few oscillations. If the needle is 

 again moved, so that it is placed considerably below the centre 

 of gravity, the slightest force will throw the beam on either 

 side, and it will not oscillate, but remain fixed as soon as the 

 centre of gravity has reached its lowest point. 



This sort of explanation is dry to read, but may readily be 

 made interesting by the performance of the experiments 

 described. In making scales, two things have specially to be 



