2IO METHODS IN TEACHING 



molasses, and mercury. Test with the hydrometer. Test 

 mercury with shot, pieces of iron, and pebbles. 



Into a test-tube about six inches long and one inch in 

 diameter put equal amounts of water, mercury, and oil. 

 Do the liquids mix? Drop a buck-shot or other small 

 piece of metal into the tube. Note that it sinks in both 

 the water and the oil but that it floats on the mercury. By 

 careful trial find a substance that floats on water but which 

 is too heavy to float on oil ; and, finally, find some substance 

 that is light enough to float on gasoline. Cork the tube 

 tightly, invert it, and note that the liquids rearrange them- 

 selves according to their several weights. Demonstrate by 

 means of the hydrometer that strong salt water is heavier 

 than fresh water. Show the same fact by actually weigh- 

 ing equal amounts. 



Water weighs about sixty-two and a half pounds to the 

 cubic foot. In a tank, ten feet deep, what is the pressure 

 on each square foot of the bottom when the tank is filled 

 with water? Would a fish near the bottom of the tank 

 be under a greater pressure than one near the top ? Divers 

 can work successfully only in comparatively shallow waters, 

 from fifteen to thirty-five or forty feet deep. Why? Why 

 are tanks for private water supply usually placed high above 

 the ground? Have you ever seen standpipes used in con- 

 nection with city water systems? Why are they used? 



We live at the bottom of a great sea of air which is vari- 

 ously estimated at from fifty to four hundred miles deep. 

 The pressure at the bottom is equal to the 

 water pressure at the bottom of a tank 

 about thirty feet deep. Our bodies, developing under this 

 constant pressure, are not only used to the tremendous 



