H4 OUR PHYSICAL WORLD 



The increasing pressure of the water drives in the rubber dia- 

 phragm, exerts pressure on the air in the apparatus which forces 

 the red ink to move. Note the depth to which the thistle tube 

 has been sunk and also the position of the red ink against the 

 scale. * Now place the diaphragm of the thistle tube at the same 

 depth in the water but turned up instead of down and note the 

 pressure. Let it be turned sideways at the same depth. In all 

 these positions the drop of red ink in the small tube will register 

 the same on the scale, showing that the pressure of the liquid is 

 the same in all directions. 



Having given the child now some notion of the nature of 

 gas and an appreciation of fluid pressure and the fact that the 

 pressure is exerted equally in all directions, we must next give 

 some conception of what happens when a body is immersed in a 

 fluid. Cut from a block of plasticine a piece i centimeter wide, 

 i centimeter thick, and 5 centimeters long. Cut this as accu- 

 rately as possible. Fasten a piece of thread to this so that it 

 may be lowered into a glass graduate of 100 cubic centimeters 

 capacity. Fill the graduate up to the 50 cubic-centimeter 

 mark with water and then lower into this the piece of plasticine. 

 The water in the graduate will now rise to the 55 cubic- 

 centimeter point, and, since the block of plasticine contained 

 5 cubic centimeters, it is evident that a body immersed in water 

 displaces its own volume of water. Withdraw the block of 

 plasticine and press it out of its regular shape between the fingers, 

 then lower it again into the water. We still, of course, have 

 5 cubic centimeters of plasticine in the block, and it will still dis- 

 place the same volume of water, but now the child knows that 

 this law holds true even with irregular objects. 



The history of the discovery of this law is interesting. A 

 certain king of Greece had given to his artificers of metal a lump 

 of gold which was to be made into a crown. The king suspected 

 that his workmen had abstracted some of the gold and that the 

 crown was made in part of silver a much less valuable metal- 

 which had been substituted for the gold by his craftsmen. He 



