PRINCIPLE OF ARCHIMEDES 



51 



Buoyancy. Most people have noticed when in a bath that if there 

 is water enough, and they take hold of no support, the water buoys 

 them up, or they experience a tendency to rise up through the water. 

 It is as if the water resists being displaced and presses the displacing 

 object upwards. In the case of things which float, such as a wooden 

 rod or a lead pencil, the results of this upthrust which the water exerts 

 can easily be seen, by pushing either the rod or pencil down into the 



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PIG. 41. How to find the weight of an object suspended in water. 



water and then letting go, when the solid rises up through the water. 

 Even in the case of bodies which sink, there is the same upthrust on 

 the part of the water, but it is not enough to float them. The effect 

 which the water has upon such bodies can, however, be seen in the loss 

 of weight which they experience if they are weighed by a spring balance 

 when hanging in water (Fig. 40). 



. Loss of weight of things immersed in water. It is easy to prove 

 by experiment that an object weighs less in water than out of it. If 

 a cubic centimetre of lead or any other heavy material, is hung from a 

 spring balance and then suspended in water, it will be found to weigh 

 the weight of one gram less in water than out. If two cubic centi- 

 metres are suspended from the balance, the loss of weight is the weight 

 of two grams. In every case the loss of weight measured in this way 

 is equal to the number of cubic centimetres of the solid immersed in 

 the water. The loss is thus equal to the weight of the water displaced. 

 This fact leads to a highly important conclusion, known after its dis- 

 coverer as the Principle of Archimedes. 



The Principle of Archimedes. When a body is immersed in water 

 it loses weight equal to the weight of the water displaced by it. If 



