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and will be ajjpreciated. It is best not to have the rod loaded ready for 

 the experiment, but reserve for the class the loading the rod just right 

 to make it float upright in the two liquids, and marking the scale on it. 

 With this now ready have them test the liquids. They find that the rod 

 floats higher in the heavier liquid. Now make mixtures 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 ; such as 

 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 hydrometer (see physics), can be bor- 

 rowed to show, and can be made use of, the value of the lesson will be 

 greater. But they will see that none of them are 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 them that it is "the weight of the water dis- 

 placed." This is misleading and once given seems to become a sort of 

 cant phrase in which it is hard to put the real meaning. Later this 

 truth may be seen and verified. While the whole mathematical explana- 

 tion can not be gone into, the simple fact that floating is due to the up- 

 ward pressure of the water may be clearly seen. 



Show them that when a body is placed in the water that the water 

 presses against the whole surface. A bucket pushed down would show 

 the presEure 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. 



The children will readily see that the pressure of a liquid is due to its 

 weight, and consequently is greater in the heavier liquids. It will be 

 very interesting to show a very light liquid like gasoline (do not use it 

 near a light) and the heavy mercury. 



From liquids it is easy to pass to air. In previous lessons air has been 

 shown to be "something," and consequently has weight and exerts 

 pressure. Let the children devise means of showing this to be true. 



Put experiments like this in the form of questions: A glass of water 

 evenly full is covered with a piece of paper, and then suddenly inverted, 

 the paper being held on by the palm of the hand. When the hand is 

 removed the water will be kept in the glass by the pressure of the 



