Pleasant Ways in Science. 343 



the task is found to be surrounded by a variety of difficulties, 

 requiring considerable skill and science to overcome. To 

 understand this, wo must arrive first at a distinct compre- 

 hension of what is meant by centre of gravity. Terrestrial 

 gravity, or weight, means the mutual attraction exerted 

 between the earth and any given body, as, for example, a piece 

 of wood. Take a strip of wood, or of card, which will do as 

 well, say six inches long, and half an inch wide, run a needle 

 through it near one end, and support the needle on a couple of 

 wine-glasses, so that the card can fall between them. The 

 long end of the card will touch the table on which the glasses 

 stand, and, if lifted up, will immediately fall back again. 

 Why ? Because the earth attracts and is attracted by all the 

 particles of the card, and there are more of them in the long 

 end than in the short ; that end therefore falls. If the short 

 end is weighted, so as to attract and be attracted by the earth as 

 much as the long end, a balance will be obtained, and neither 

 end will fall. The quantity of weight on either side of the 

 needle which passes through the point of suspension can be 

 adjusted quite as well by moving the needle as by adding to 

 the weight of the lightest end. There must be a point in 

 every solid so situated that exactly as much weight lies on one 

 side of it as on the other. In a regular solid, like a solid 

 square or a solid parallelogram, we can find this point by 

 drawing diameters across opposite corners ; they will cross in 

 the centre of the figure, which is also its centre of gravity. 

 In a circle, the centre of the figure is likewise the centre of 

 gravity, presuming always that the object is of equal density 

 throughout. In irregular figures, the centre of gravity is 

 more troublesome to find; but when found, has the same 

 property, that if the object is suspended at that point it can 

 remain at rest. A body acts as if all its weight were concen- 

 trated in its centre of gravity ; and, consequently, whatever be 

 the mode of its suspension, the centre of gravity will fall as 

 low — that is, as near the centre of the earth — as it can. Now, 

 if a body has a pin run exactly through its centre of gravity, 

 and that pin is strong enough to bear its entire weight, it is 

 obvious that the centre of gravity cannot fall lower than it is 

 already placed. To do so, it would have to break or bend the 

 pin, which we have supposed impossible. If, therefore, we run 

 the needle exactly through the centre of gravity of our piece 

 of card, we shall find that it can be at rest, or balanced in any 

 position. Both arms may be horizontal or vertical, or in any 

 intermediate position ; and whatever tendency of weight operates 

 upon one arm in one direction, must operate upon the other 

 arm in exactly an opposite direction, and so both arms will be 

 in equilibrium wherever they are placed. 



