32 SCIENTIFIC RECREATIONS. 



and brought directly under the point of suspension. Fig. 26 shows the 

 method of operation very plainly. It is well to have at hand several pieces 

 of straw perfectly intact, and free from cracks, in case the experiment does 

 not succeed with the first attempt. 



Having now seen how this point we call the centre of gravity acts, we 

 may briefly explain it. 



The centre of gravity of a body is that point in which the sum of the 

 forces of gravity, acting upon all the particles, may be said to be united. We 

 know the attraction of the earth causes bodies to have a property we call 

 Weight. This property of weight presses upon every particle of the body, 



Fig. 26. Lifting a bottle with a single straw. 



and acts upon them as parallel forces. For if a stone be broken all the portions 

 will equal the weight of the stone ; and if some of them be suspended, it will 

 be seen that they hang parallel to each other, so we may call these weights 

 parallel forces united in the whole stone, and equal to a single resultant. 

 Now, to find the centre of gravity, we must suspend the body, and it will 

 hang in a certain direction. Draw a line from the point of suspension, and 

 suspend the body again : a line drawn from that point of suspension will 

 pass through the same place as the former line did, and so on. That point 

 is the centre of gravity of that suspended body. If the form of it be regular, 

 like a ball or cylinder, the centre of gravity is the same as the mathematically 

 central point. In such forms as pyramids it will be found near the largest 



