172 NATURE STUDY 



the rod floats higher in the heavier liquid. Now make the mix- 

 tures of the water and salt solution, and have them predict how 

 the rod will stand, then verify the prediction. 



Now other liquids may be tested and compared with 

 water; e. g. milk, kerosene, or a solution of salts other than 

 table salt. 



The apparatus may be varied by preparing other forms of 

 floats, e. g., a long, narrow test tube with sand, shot, or nails in 

 the bottom to act as ballast. In the tube may be placed a paper 

 scale. If a lactometer or alcoholometer, or other form of hydro, 

 meter (see text-book on physics) can be borrowed to show, and can 

 be made use of, the value of the lesson will be greater. But the 

 pupils will see that none of these instruments is anything more 

 than their wooden rod with a scale. 



An egg will float in strong brine and sink in water, and thus 

 is used as a hydrometer. These experiments extended bring the 

 phenomena of floating clearly before the pupils. 



Next the question of what makes the bodies float may be 

 taken up. At the start do not tell the pupils that it is "the weight of 

 the water displaced." This is misleading, and, once given, seems 

 to become a sort of cant phrase into which it is hard to put the 

 real meaning. Later this truth may be seen and verified. While 

 the whole mathematical explanation can not be gone into, the 

 simple fact that floating is due to the upward pressure of the 

 water may be clearly seen. 



Show the pupils that when a body is placed in the water, the 

 water presses against the whole surface. A bucket pushed down 

 would show the pressure if holes were bored in the bottom and 

 sides. A rubber boot placed in a bucket shows the pressure by 

 the collapsing of the sides. A bottle filled with air thrust mouth 

 down will show that the air is pressed upon by the water. In a 

 floating body the pressure against the sides takes no part in 

 holding the body up, but only the upward pressure does so. 



