Transportation on Water 341 



unit volume, or as we say, weight per cubic inch. As gold was 

 the densest metal the ancient Greeks knew, it was easy to detect 

 the presence of any lighter metal in the crown. The king, 

 satisfied with the proof, conferred great honor upon Archimedes 

 for his discovery. Archimedes' method may be employed to 

 find out the weight of a unit volume, or the density, of a piece 

 of iron, lead, or rock. For large pieces the spring balance is 

 fairly accurate ; but the beam balance is better for weighing in 

 air and in water. 



Exercise : Buoyancy and pressure. It is now evident that the 

 water exerts a buoyant force. Why is this the case? Punch a 

 number of holes at different levels in a tin can. Then press the can 

 into the water. Does it appear that the water presses in all directions? 

 Is the pressure on bottom and sides equal, so far as you can judge from 

 the force of the flow through the holes? When the can is full of 

 water, lift it and observe how the pressure is exerted. Is it in all 

 directions? If there is a difference in pressure, where is it? What 

 seems to cause this difference? Is there more water above or below 

 the places of greater pressure? Where does a swimmer when diving 

 experience the greater pressure on the ear drums, near the surface or 

 at a depth below? 



Put the can into the water at any definite depth. When does the 

 water cease to flow through the holes? When the water is completely 

 at rest, what is indicated in regard to pressure? Is the pressure in 

 the can equal to that outside it ? Suppose the pressures were unequal, 

 would the water remain at rest or continue in motion ? What is the 

 relation of the levels of the water inside and outside the can, so that 

 the pressure at any one of the holes shall be equal in both directions? 

 What difference would you expect to find if you had pressed the can 

 into a large body of water as a river or lake? Does pressure depend 

 upon the bulk of water above the hole, or upon the distance of the 

 hole below the water's surface? 



Pressure and depth. As shown in the study on water pres- 

 sure under the topic of water supply (page 227) pressure varies 

 with the depth (Fig. 116). This indicates that it is the weight of 

 a column of water above a given place, which determines the pres- 

 sure. It is well known that in deep parts of the ocean, fish and 

 plants are peculiarly adapted to withstand enormous pressure and 



